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<section id="inter-vm-shared-memory-device">
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<h1>Inter-VM Shared Memory device<a class="headerlink" href="#inter-vm-shared-memory-device" title="Link to this heading"></a></h1>
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<p>On Linux hosts, a shared memory device is available. The basic syntax
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is:</p>
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<pre class="literal-block">qemu-system-x86_64 -device ivshmem-plain,memdev=hostmem</pre>
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<p>where hostmem names a host memory backend. For a POSIX shared memory
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backend, use something like</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="nb">object</span> <span class="n">memory</span><span class="o">-</span><span class="n">backend</span><span class="o">-</span><span class="n">file</span><span class="p">,</span><span class="n">size</span><span class="o">=</span><span class="mi">1</span><span class="n">M</span><span class="p">,</span><span class="n">share</span><span class="p">,</span><span class="n">mem</span><span class="o">-</span><span class="n">path</span><span class="o">=/</span><span class="n">dev</span><span class="o">/</span><span class="n">shm</span><span class="o">/</span><span class="n">ivshmem</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">hostmem</span>
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</pre></div>
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</div>
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<p>If desired, interrupts can be sent between guest VMs accessing the same
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shared memory region. Interrupt support requires using a shared memory
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server and using a chardev socket to connect to it. The code for the
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shared memory server is qemu.git/contrib/ivshmem-server. An example
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syntax when using the shared memory server is:</p>
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<pre class="literal-block"># First start the ivshmem server once and for all
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ivshmem-server -p pidfile -S path -m shm-name -l shm-size -n vectors
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# Then start your qemu instances with matching arguments
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qemu-system-x86_64 -device ivshmem-doorbell,vectors=vectors,chardev=id
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-chardev socket,path=path,id=id</pre>
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<p>When using the server, the guest will be assigned a VM ID (>=0) that
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allows guests using the same server to communicate via interrupts.
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Guests can read their VM ID from a device register (see
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<a class="reference internal" href="../../specs/ivshmem-spec.html"><span class="doc">Device Specification for Inter-VM shared memory device</span></a>).</p>
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<section id="migration-with-ivshmem">
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<h2>Migration with ivshmem<a class="headerlink" href="#migration-with-ivshmem" title="Link to this heading"></a></h2>
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<p>With device property <code class="docutils literal notranslate"><span class="pre">master=on</span></code>, the guest will copy the shared
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memory on migration to the destination host. With <code class="docutils literal notranslate"><span class="pre">master=off</span></code>, the
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guest will not be able to migrate with the device attached. In the
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latter case, the device should be detached and then reattached after
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migration using the PCI hotplug support.</p>
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<p>At most one of the devices sharing the same memory can be master. The
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master must complete migration before you plug back the other devices.</p>
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</section>
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<section id="ivshmem-and-hugepages">
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<h2>ivshmem and hugepages<a class="headerlink" href="#ivshmem-and-hugepages" title="Link to this heading"></a></h2>
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<p>Instead of specifying the <shm size> using POSIX shm, you may specify a
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memory backend that has hugepage support:</p>
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<pre class="literal-block">qemu-system-x86_64 -object memory-backend-file,size=1G,mem-path=/dev/hugepages/my-shmem-file,share,id=mb1
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-device ivshmem-plain,memdev=mb1</pre>
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<p>ivshmem-server also supports hugepages mount points with the <code class="docutils literal notranslate"><span class="pre">-m</span></code>
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memory path argument.</p>
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