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Re: [PATCH 1/1] conf: qemu 9pfs: add 'multidevs' option

From: Ján Tomko
Subject: Re: [PATCH 1/1] conf: qemu 9pfs: add 'multidevs' option
Date: Thu, 19 Mar 2020 14:10:26 +0100

On a Tuesday in 2020, Christian Schoenebeck wrote:
Introduce new 'multidevs' option for filesystem.

 <filesystem type='mount' accessmode='mapped' multidevs='remap'>

I don't like the 'multidevs' name, but cannot think of anything

'collisions' maybe?

   <source dir='/path'/>
   <target dir='mount_tag'>

This option prevents misbheaviours on guest if a 9pfs export
contains multiple devices, due to the potential file ID collisions
this otherwise may cause.

Signed-off-by: Christian Schoenebeck <address@hidden>
docs/formatdomain.html.in     | 47 ++++++++++++++++++++++++++++++++++-
docs/schemas/domaincommon.rng | 10 ++++++++
src/conf/domain_conf.c        | 30 ++++++++++++++++++++++
src/conf/domain_conf.h        | 13 ++++++++++
src/qemu/qemu_command.c       |  7 ++++++
5 files changed, 106 insertions(+), 1 deletion(-)

Please split the XML changes from the qemu driver changes.

Also missing:
* qemu_capabilities addition
* qemuDomainDeviceDefValidateFS in qemu_domain.c - check for the capability,
  reject this setting for virtiofs
* qemuxml2xmltest addition
* qemuxml2argvtest addition

(no changes required for virschematest - it checks all the XML files in
the directories used by the above tests against the schema)

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 594146009d..13c506988b 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -3967,7 +3967,7 @@
    &lt;source name='my-vm-template'/&gt;
    &lt;target dir='/'/&gt;
-  &lt;filesystem type='mount' accessmode='passthrough'&gt;
+  &lt;filesystem type='mount' accessmode='passthrough' multidevs='remap'&gt;
    &lt;driver type='path' wrpolicy='immediate'/&gt;
    &lt;source dir='/export/to/guest'/&gt;
    &lt;target dir='/import/from/host'/&gt;
@@ -4084,13 +4084,58 @@

+      <p>
      <span class="since">Since 5.2.0</span>, the filesystem element
      has an optional attribute <code>model</code> with supported values
      "virtio-transitional", "virtio-non-transitional", or "virtio".
      See <a href="#elementsVirtioTransitional">Virtio transitional devices</a>
      for more details.
+      </p>

Unrelated change that can be split out.

+      <p>
+      The filesystem element has an optional attribute <code>multidevs</code>
+      which specifies how to deal with a filesystem export containing more than
+      one device, in order to avoid file ID collisions on guest when using 9pfs
+      (<span class="since">since 6.2.0, requires QEMU 4.2</span>).
+      This attribute is not available for virtiofs. The possible values are:
+      </p>
+        <dl>
+        <dt><code>default</code></dt>
+        <dd>
+        Use QEMU's default setting (which currently is <code>warn</code>).
+        </dd>
+        <dt><code>remap</code></dt>
+        <dd>
+        This setting allows guest to access multiple devices per export without
+        encountering misbehaviours. Inode numbers from host are automatically
+        remapped on guest to actively prevent file ID collisions if guest
+        accesses one export containing multiple devices.
+        </dd>
+        <dt><code>forbid</code></dt>
+        <dd>
+        Only allow to access one device per export by guest. Attempts to access
+        additional devices on the same export will cause the individual
+        filesystem access by guest to fail with an error and being logged 
+        as error on host side.
+        </dd>
+        <dt><code>warn</code></dt>
+        <dd>
+        This setting resembles the behaviour of 9pfs prior to QEMU 4.2, that is
+        no action is performed to prevent any potential file ID collisions if 
+        export contains multiple devices, with the only exception: a warning is
+        logged (once) on host side now. This setting may lead to misbehaviours
+        on guest side if more than one device is exported per export, due to 
+        potential file ID collisions this may cause on guest side in that case.
+        </dd>
+        </dl>

+      <p>
+      The <code>filesystem</code> element may contain the following 
+      </p>

And so can this one.

        The optional driver element allows specifying further details
@@ -25422,6 +25449,9 @@ virDomainFSDefFormat(virBufferPtr buf,
        virBufferAsprintf(buf, " model='%s'",
+    if (def->multidevs) {
+        virBufferAsprintf(buf, " multidevs='%s'", multidevs);
+    }

make syntax-check complains here:
Curly brackets around single-line body:
    if (def->multidevs) {
        virBufferAsprintf(buf, " multidevs='%s'", multidevs);


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