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<section id="amd-secure-encrypted-virtualization-sev">
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<h1>AMD Secure Encrypted Virtualization (SEV)<a class="headerlink" href="#amd-secure-encrypted-virtualization-sev" title="Link to this heading"></a></h1>
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<p>Secure Encrypted Virtualization (SEV) is a feature found on AMD processors.</p>
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<p>SEV is an extension to the AMD-V architecture which supports running encrypted
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virtual machines (VMs) under the control of KVM. Encrypted VMs have their pages
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(code and data) secured such that only the guest itself has access to the
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unencrypted version. Each encrypted VM is associated with a unique encryption
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key; if its data is accessed by a different entity using a different key the
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encrypted guests data will be incorrectly decrypted, leading to unintelligible
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data.</p>
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<p>Key management for this feature is handled by a separate processor known as the
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AMD secure processor (AMD-SP), which is present in AMD SOCs. Firmware running
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inside the AMD-SP provides commands to support a common VM lifecycle. This
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includes commands for launching, snapshotting, migrating and debugging the
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encrypted guest. These SEV commands can be issued via KVM_MEMORY_ENCRYPT_OP
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ioctls.</p>
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<p>Secure Encrypted Virtualization - Encrypted State (SEV-ES) builds on the SEV
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support to additionally protect the guest register state. In order to allow a
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hypervisor to perform functions on behalf of a guest, there is architectural
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support for notifying a guest’s operating system when certain types of VMEXITs
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are about to occur. This allows the guest to selectively share information with
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the hypervisor to satisfy the requested function.</p>
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<section id="launching">
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<h2>Launching<a class="headerlink" href="#launching" title="Link to this heading"></a></h2>
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<p>Boot images (such as bios) must be encrypted before a guest can be booted. The
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<code class="docutils literal notranslate"><span class="pre">MEMORY_ENCRYPT_OP</span></code> ioctl provides commands to encrypt the images: <code class="docutils literal notranslate"><span class="pre">LAUNCH_START</span></code>,
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<code class="docutils literal notranslate"><span class="pre">LAUNCH_UPDATE_DATA</span></code>, <code class="docutils literal notranslate"><span class="pre">LAUNCH_MEASURE</span></code> and <code class="docutils literal notranslate"><span class="pre">LAUNCH_FINISH</span></code>. These four commands
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together generate a fresh memory encryption key for the VM, encrypt the boot
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images and provide a measurement than can be used as an attestation of a
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successful launch.</p>
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<p>For a SEV-ES guest, the <code class="docutils literal notranslate"><span class="pre">LAUNCH_UPDATE_VMSA</span></code> command is also used to encrypt the
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guest register state, or VM save area (VMSA), for all of the guest vCPUs.</p>
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<p><code class="docutils literal notranslate"><span class="pre">LAUNCH_START</span></code> is called first to create a cryptographic launch context within
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the firmware. To create this context, guest owner must provide a guest policy,
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its public Diffie-Hellman key (PDH) and session parameters. These inputs
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should be treated as a binary blob and must be passed as-is to the SEV firmware.</p>
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<p>The guest policy is passed as plaintext. A hypervisor may choose to read it,
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but should not modify it (any modification of the policy bits will result
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in bad measurement). The guest policy is a 4-byte data structure containing
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several flags that restricts what can be done on a running SEV guest.
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See SEV API Spec (<a class="reference internal" href="#sevapi" id="id1"><span>[SEVAPI]</span></a>) section 3 and 6.2 for more details.</p>
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<p>The guest policy can be provided via the <code class="docutils literal notranslate"><span class="pre">policy</span></code> property:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># ${QEMU} \</span>
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<span class="n">sev</span><span class="o">-</span><span class="n">guest</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">sev0</span><span class="p">,</span><span class="n">policy</span><span class="o">=</span><span class="mh">0x1</span><span class="o">...</span>\
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</pre></div>
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</div>
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<p>Setting the “SEV-ES required” policy bit (bit 2) will launch the guest as a
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SEV-ES guest:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># ${QEMU} \</span>
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<span class="n">sev</span><span class="o">-</span><span class="n">guest</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">sev0</span><span class="p">,</span><span class="n">policy</span><span class="o">=</span><span class="mh">0x5</span><span class="o">...</span>\
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</pre></div>
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</div>
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<p>The guest owner provided DH certificate and session parameters will be used to
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establish a cryptographic session with the guest owner to negotiate keys used
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for the attestation.</p>
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<p>The DH certificate and session blob can be provided via the <code class="docutils literal notranslate"><span class="pre">dh-cert-file</span></code> and
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<code class="docutils literal notranslate"><span class="pre">session-file</span></code> properties:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># ${QEMU} \</span>
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<span class="n">sev</span><span class="o">-</span><span class="n">guest</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">sev0</span><span class="p">,</span><span class="n">dh</span><span class="o">-</span><span class="n">cert</span><span class="o">-</span><span class="n">file</span><span class="o">=<</span><span class="n">file1</span><span class="o">></span><span class="p">,</span><span class="n">session</span><span class="o">-</span><span class="n">file</span><span class="o">=<</span><span class="n">file2</span><span class="o">></span>
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</pre></div>
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</div>
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<p><code class="docutils literal notranslate"><span class="pre">LAUNCH_UPDATE_DATA</span></code> encrypts the memory region using the cryptographic context
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created via the <code class="docutils literal notranslate"><span class="pre">LAUNCH_START</span></code> command. If required, this command can be called
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multiple times to encrypt different memory regions. The command also calculates
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the measurement of the memory contents as it encrypts.</p>
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<p><code class="docutils literal notranslate"><span class="pre">LAUNCH_UPDATE_VMSA</span></code> encrypts all the vCPU VMSAs for a SEV-ES guest using the
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cryptographic context created via the <code class="docutils literal notranslate"><span class="pre">LAUNCH_START</span></code> command. The command also
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calculates the measurement of the VMSAs as it encrypts them.</p>
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<p><code class="docutils literal notranslate"><span class="pre">LAUNCH_MEASURE</span></code> can be used to retrieve the measurement of encrypted memory and,
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for a SEV-ES guest, encrypted VMSAs. This measurement is a signature of the
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memory contents and, for a SEV-ES guest, the VMSA contents, that can be sent
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||
to the guest owner as an attestation that the memory and VMSAs were encrypted
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||
correctly by the firmware. The guest owner may wait to provide the guest
|
||
confidential information until it can verify the attestation measurement.
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||
Since the guest owner knows the initial contents of the guest at boot, the
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||
attestation measurement can be verified by comparing it to what the guest owner
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||
expects.</p>
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<p><code class="docutils literal notranslate"><span class="pre">LAUNCH_FINISH</span></code> finalizes the guest launch and destroys the cryptographic
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context.</p>
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<p>See SEV API Spec (<a class="reference internal" href="#sevapi" id="id2"><span>[SEVAPI]</span></a>) ‘Launching a guest’ usage flow (Appendix A) for the
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complete flow chart.</p>
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<p>To launch a SEV guest:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># ${QEMU} \</span>
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<span class="o">-</span><span class="n">machine</span> <span class="o">...</span><span class="p">,</span><span class="n">confidential</span><span class="o">-</span><span class="n">guest</span><span class="o">-</span><span class="n">support</span><span class="o">=</span><span class="n">sev0</span> \
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<span class="o">-</span><span class="nb">object</span> <span class="n">sev</span><span class="o">-</span><span class="n">guest</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">sev0</span><span class="p">,</span><span class="n">cbitpos</span><span class="o">=</span><span class="mi">47</span><span class="p">,</span><span class="n">reduced</span><span class="o">-</span><span class="n">phys</span><span class="o">-</span><span class="n">bits</span><span class="o">=</span><span class="mi">1</span>
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</pre></div>
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</div>
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<p>To launch a SEV-ES guest:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># ${QEMU} \</span>
|
||
<span class="o">-</span><span class="n">machine</span> <span class="o">...</span><span class="p">,</span><span class="n">confidential</span><span class="o">-</span><span class="n">guest</span><span class="o">-</span><span class="n">support</span><span class="o">=</span><span class="n">sev0</span> \
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||
<span class="o">-</span><span class="nb">object</span> <span class="n">sev</span><span class="o">-</span><span class="n">guest</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">sev0</span><span class="p">,</span><span class="n">cbitpos</span><span class="o">=</span><span class="mi">47</span><span class="p">,</span><span class="n">reduced</span><span class="o">-</span><span class="n">phys</span><span class="o">-</span><span class="n">bits</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span><span class="n">policy</span><span class="o">=</span><span class="mh">0x5</span>
|
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</pre></div>
|
||
</div>
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<p>An SEV-ES guest has some restrictions as compared to a SEV guest. Because the
|
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guest register state is encrypted and cannot be updated by the VMM/hypervisor,
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a SEV-ES guest:</p>
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<blockquote>
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<div><ul class="simple">
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<li><p>Does not support SMM - SMM support requires updating the guest register
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state.</p></li>
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<li><p>Does not support reboot - a system reset requires updating the guest register
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state.</p></li>
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<li><p>Requires in-kernel irqchip - the burden is placed on the hypervisor to
|
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manage booting APs.</p></li>
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</ul>
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</div></blockquote>
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</section>
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<section id="calculating-expected-guest-launch-measurement">
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<h2>Calculating expected guest launch measurement<a class="headerlink" href="#calculating-expected-guest-launch-measurement" title="Link to this heading"></a></h2>
|
||
<p>In order to verify the guest launch measurement, The Guest Owner must compute
|
||
it in the exact same way as it is calculated by the AMD-SP. SEV API Spec
|
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(<a class="reference internal" href="#sevapi" id="id3"><span>[SEVAPI]</span></a>) section 6.5.1 describes the AMD-SP operations:</p>
|
||
<blockquote>
|
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<div><p>GCTX.LD is finalized, producing the hash digest of all plaintext data
|
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imported into the guest.</p>
|
||
<p>The launch measurement is calculated as:</p>
|
||
<p>HMAC(0x04 || API_MAJOR || API_MINOR || BUILD || GCTX.POLICY || GCTX.LD || MNONCE; GCTX.TIK)</p>
|
||
<p>where “||” represents concatenation.</p>
|
||
</div></blockquote>
|
||
<p>The values of API_MAJOR, API_MINOR, BUILD, and GCTX.POLICY can be obtained
|
||
from the <code class="docutils literal notranslate"><span class="pre">query-sev</span></code> qmp command.</p>
|
||
<p>The value of MNONCE is part of the response of <code class="docutils literal notranslate"><span class="pre">query-sev-launch-measure</span></code>: it
|
||
is the last 16 bytes of the base64-decoded data field (see SEV API Spec
|
||
(<a class="reference internal" href="#sevapi" id="id4"><span>[SEVAPI]</span></a>) section 6.5.2 Table 52: LAUNCH_MEASURE Measurement Buffer).</p>
|
||
<p>The value of GCTX.LD is
|
||
<code class="docutils literal notranslate"><span class="pre">SHA256(firmware_blob</span> <span class="pre">||</span> <span class="pre">kernel_hashes_blob</span> <span class="pre">||</span> <span class="pre">vmsas_blob)</span></code>, where:</p>
|
||
<ul class="simple">
|
||
<li><p><code class="docutils literal notranslate"><span class="pre">firmware_blob</span></code> is the content of the entire firmware flash file (for
|
||
example, <code class="docutils literal notranslate"><span class="pre">OVMF.fd</span></code>). Note that you must build a stateless firmware file
|
||
which doesn’t use an NVRAM store, because the NVRAM area is not measured, and
|
||
therefore it is not secure to use a firmware which uses state from an NVRAM
|
||
store.</p></li>
|
||
<li><p>if kernel is used, and <code class="docutils literal notranslate"><span class="pre">kernel-hashes=on</span></code>, then <code class="docutils literal notranslate"><span class="pre">kernel_hashes_blob</span></code> is
|
||
the content of PaddedSevHashTable (including the zero padding), which itself
|
||
includes the hashes of kernel, initrd, and cmdline that are passed to the
|
||
guest. The PaddedSevHashTable struct is defined in <code class="docutils literal notranslate"><span class="pre">target/i386/sev.c</span></code>.</p></li>
|
||
<li><p>if SEV-ES is enabled (<code class="docutils literal notranslate"><span class="pre">policy</span> <span class="pre">&</span> <span class="pre">0x4</span> <span class="pre">!=</span> <span class="pre">0</span></code>), <code class="docutils literal notranslate"><span class="pre">vmsas_blob</span></code> is the
|
||
concatenation of all VMSAs of the guest vcpus. Each VMSA is 4096 bytes long;
|
||
its content is defined inside Linux kernel code as <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">vmcb_save_area</span></code>,
|
||
or in AMD APM Volume 2 (<a class="reference internal" href="#apmvol2" id="id5"><span>[APMVOL2]</span></a>) Table B-2: VMCB Layout, State Save Area.</p></li>
|
||
</ul>
|
||
<p>If kernel hashes are not used, or SEV-ES is disabled, use empty blobs for
|
||
<code class="docutils literal notranslate"><span class="pre">kernel_hashes_blob</span></code> and <code class="docutils literal notranslate"><span class="pre">vmsas_blob</span></code> as needed.</p>
|
||
</section>
|
||
<section id="debugging">
|
||
<h2>Debugging<a class="headerlink" href="#debugging" title="Link to this heading"></a></h2>
|
||
<p>Since the memory contents of a SEV guest are encrypted, hypervisor access to
|
||
the guest memory will return cipher text. If the guest policy allows debugging,
|
||
then a hypervisor can use the DEBUG_DECRYPT and DEBUG_ENCRYPT commands to access
|
||
the guest memory region for debug purposes. This is not supported in QEMU yet.</p>
|
||
</section>
|
||
<section id="snapshot-restore">
|
||
<h2>Snapshot/Restore<a class="headerlink" href="#snapshot-restore" title="Link to this heading"></a></h2>
|
||
<p>TODO</p>
|
||
</section>
|
||
<section id="live-migration">
|
||
<h2>Live Migration<a class="headerlink" href="#live-migration" title="Link to this heading"></a></h2>
|
||
<p>TODO</p>
|
||
</section>
|
||
<section id="references">
|
||
<h2>References<a class="headerlink" href="#references" title="Link to this heading"></a></h2>
|
||
<p><a class="reference external" href="https://www.amd.com/content/dam/amd/en/documents/epyc-business-docs/white-papers/memory-encryption-white-paper.pdf">AMD Memory Encryption whitepaper</a></p>
|
||
<div role="list" class="citation-list">
|
||
<div class="citation" id="sevapi" role="doc-biblioentry">
|
||
<span class="label"><span class="fn-bracket">[</span>SEVAPI<span class="fn-bracket">]</span></span>
|
||
<span class="backrefs">(<a role="doc-backlink" href="#id1">1</a>,<a role="doc-backlink" href="#id2">2</a>,<a role="doc-backlink" href="#id3">3</a>,<a role="doc-backlink" href="#id4">4</a>)</span>
|
||
<p><a class="reference external" href="https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf">Secure Encrypted Virtualization API</a></p>
|
||
</div>
|
||
<div class="citation" id="apmvol2" role="doc-biblioentry">
|
||
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id5">APMVOL2</a><span class="fn-bracket">]</span></span>
|
||
<p><a class="reference external" href="https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf">AMD64 Architecture Programmer’s Manual Volume 2: System Programming</a></p>
|
||
</div>
|
||
</div>
|
||
<p>KVM Forum slides:</p>
|
||
<ul class="simple">
|
||
<li><p><a class="reference external" href="http://www.linux-kvm.org/images/7/74/02x08A-Thomas_Lendacky-AMDs_Virtualizatoin_Memory_Encryption_Technology.pdf">AMD’s Virtualization Memory Encryption (2016)</a></p></li>
|
||
<li><p><a class="reference external" href="https://www.linux-kvm.org/images/9/94/Extending-Secure-Encrypted-Virtualization-with-SEV-ES-Thomas-Lendacky-AMD.pdf">Extending Secure Encrypted Virtualization With SEV-ES (2018)</a></p></li>
|
||
</ul>
|
||
<p><a class="reference external" href="https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf">AMD64 Architecture Programmer’s Manual:</a></p>
|
||
<ul class="simple">
|
||
<li><p>SME is section 7.10</p></li>
|
||
<li><p>SEV is section 15.34</p></li>
|
||
<li><p>SEV-ES is section 15.35</p></li>
|
||
</ul>
|
||
</section>
|
||
</section>
|
||
|
||
|
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