Introduce new 'multidevs' option for filesystem.

  <filesystem type='mount' accessmode='mapped' multidevs='remap'>
    <source dir='/path'/>
    <target dir='mount_tag'>
  </filesystem>

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

Signed-off-by: Christian Schoenebeck <qemu_...@crudebyte.com>
---
 docs/formatdomain.html.in                     | 40 ++++++++++++-
 docs/schemas/domaincommon.rng                 | 10 ++++
 src/conf/domain_conf.c                        | 29 ++++++++++
 src/conf/domain_conf.h                        | 13 +++++
 .../qemuxml2argvdata/virtio-9p-multidevs.xml  | 53 ++++++++++++++++++
 .../virtio-9p-multidevs.x86_64-latest.xml     | 56 +++++++++++++++++++
 tests/qemuxml2xmltest.c                       |  2 +
 7 files changed, 202 insertions(+), 1 deletion(-)
 create mode 100644 tests/qemuxml2argvdata/virtio-9p-multidevs.xml
 create mode 100644 
tests/qemuxml2xmloutdata/virtio-9p-multidevs.x86_64-latest.xml

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 28e779b90a..43e8a557c0 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&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;
@@ -4092,6 +4092,44 @@
       for more details.
       </p>
 
+      <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 
(once)
+        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 
an
+        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 
the
+        potential file ID collisions this may cause on guest side in that case.
+        </dd>
+        </dl>
+
       </dd>
 
       <p>
diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
index 5de17593c1..6bfb8e8d27 100644
--- a/docs/schemas/domaincommon.rng
+++ b/docs/schemas/domaincommon.rng
@@ -2700,6 +2700,16 @@
             </choice>
           </attribute>
         </optional>
+        <optional>
+          <attribute name="multidevs">
+            <choice>
+              <value>default</value>
+              <value>remap</value>
+              <value>forbid</value>
+              <value>warn</value>
+            </choice>
+          </attribute>
+        </optional>
         <optional>
           <element name='readonly'>
             <empty/>
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 27bc5a797b..011d244ed6 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -502,6 +502,14 @@ VIR_ENUM_IMPL(virDomainFSModel,
               "virtio-non-transitional",
 );
 
+VIR_ENUM_IMPL(virDomainFSMultidevs,
+              VIR_DOMAIN_FS_MULTIDEVS_LAST,
+              "default",
+              "remap",
+              "forbid",
+              "warn",
+);
+
 VIR_ENUM_IMPL(virDomainFSCacheMode,
               VIR_DOMAIN_FS_CACHE_MODE_LAST,
               "default",
@@ -11385,6 +11393,7 @@ virDomainFSDefParseXML(virDomainXMLOptionPtr xmlopt,
     g_autofree char *usage = NULL;
     g_autofree char *units = NULL;
     g_autofree char *model = NULL;
+    g_autofree char *multidevs = NULL;
 
     ctxt->node = node;
 
@@ -11423,6 +11432,17 @@ virDomainFSDefParseXML(virDomainXMLOptionPtr xmlopt,
         }
     }
 
+    multidevs = virXMLPropString(node, "multidevs");
+    if (multidevs) {
+        if ((def->multidevs = virDomainFSMultidevsTypeFromString(multidevs)) < 
0) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("unknown multidevs '%s'"), multidevs);
+            goto error;
+        }
+    } else {
+        def->multidevs = VIR_DOMAIN_FS_MULTIDEVS_DEFAULT;
+    }
+
     if (virDomainParseScaledValue("./space_hard_limit[1]",
                                   NULL, ctxt, &def->space_hard_limit,
                                   1, ULLONG_MAX, false) < 0)
@@ -25380,6 +25400,7 @@ virDomainFSDefFormat(virBufferPtr buf,
     const char *accessmode = 
virDomainFSAccessModeTypeToString(def->accessmode);
     const char *fsdriver = virDomainFSDriverTypeToString(def->fsdriver);
     const char *wrpolicy = virDomainFSWrpolicyTypeToString(def->wrpolicy);
+    const char *multidevs = virDomainFSMultidevsTypeToString(def->multidevs);
     const char *src = def->src->path;
     g_auto(virBuffer) driverAttrBuf = VIR_BUFFER_INITIALIZER;
     g_auto(virBuffer) driverBuf = VIR_BUFFER_INIT_CHILD(buf);
@@ -25398,6 +25419,12 @@ virDomainFSDefFormat(virBufferPtr buf,
         return -1;
     }
 
+    if (!multidevs) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("unexpected multidevs %d"), def->multidevs);
+        return -1;
+    }
+
     virBufferAsprintf(buf,
                       "<filesystem type='%s' accessmode='%s'",
                       type, accessmode);
@@ -25405,6 +25432,8 @@ virDomainFSDefFormat(virBufferPtr buf,
         virBufferAsprintf(buf, " model='%s'",
                           virDomainFSModelTypeToString(def->model));
     }
+    if (def->multidevs)
+        virBufferAsprintf(buf, " multidevs='%s'", multidevs);
     virBufferAddLit(buf, ">\n");
 
     virBufferAdjustIndent(buf, 2);
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 33875d942f..2038b54c32 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -796,6 +796,18 @@ typedef enum {
     VIR_DOMAIN_FS_WRPOLICY_LAST
 } virDomainFSWrpolicy;
 
+/* How to handle exports containing multiple devices. */
+typedef enum {
+    VIR_DOMAIN_FS_MULTIDEVS_DEFAULT = 0, /* Use QEMU's default setting */
+    VIR_DOMAIN_FS_MULTIDEVS_REMAP, /* Remap inodes from host to guest */
+    VIR_DOMAIN_FS_MULTIDEVS_FORBID, /* Prohibit more than one device */
+    VIR_DOMAIN_FS_MULTIDEVS_WARN, /* Just log a warning if multiple devices */
+
+    VIR_DOMAIN_FS_MULTIDEVS_LAST
+} virDomainFSMultidevs;
+
+VIR_ENUM_DECL(virDomainFSMultidevs);
+
 typedef enum {
     VIR_DOMAIN_FS_MODEL_DEFAULT = 0,
     VIR_DOMAIN_FS_MODEL_VIRTIO,
@@ -820,6 +832,7 @@ struct _virDomainFSDef {
     int wrpolicy; /* enum virDomainFSWrpolicy */
     int format; /* virStorageFileFormat */
     int model; /* virDomainFSModel */
+    int multidevs; /* virDomainFSMultidevs */
     unsigned long long usage; /* in bytes */
     virStorageSourcePtr src;
     char *dst;
diff --git a/tests/qemuxml2argvdata/virtio-9p-multidevs.xml 
b/tests/qemuxml2argvdata/virtio-9p-multidevs.xml
new file mode 100644
index 0000000000..7a3b865415
--- /dev/null
+++ b/tests/qemuxml2argvdata/virtio-9p-multidevs.xml
@@ -0,0 +1,53 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219136</memory>
+  <currentMemory unit='KiB'>219136</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <controller type='usb' index='0'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' 
function='0x2'/>
+    </controller>
+    <controller type='ide' index='0'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' 
function='0x1'/>
+    </controller>
+    <controller type='pci' index='0' model='pci-root'/>
+    <filesystem type='mount' accessmode='mapped' multidevs='remap'>
+      <source dir='/export/fs0'/>
+      <target dir='fs0'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' 
function='0x0'/>
+    </filesystem>
+    <filesystem type='mount' accessmode='mapped' multidevs='forbid'>
+      <source dir='/export/fs1'/>
+      <target dir='fs1'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' 
function='0x0'/>
+    </filesystem>
+    <filesystem type='mount' accessmode='mapped' multidevs='warn'>
+      <source dir='/export/fs2'/>
+      <target dir='fs2'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' 
function='0x0'/>
+    </filesystem>
+    <serial type='pty'>
+      <target type='isa-serial' port='0'>
+        <model name='isa-serial'/>
+      </target>
+    </serial>
+    <console type='pty'>
+      <target type='serial' port='0'/>
+    </console>
+    <input type='mouse' bus='ps2'/>
+    <input type='keyboard' bus='ps2'/>
+    <memballoon model='virtio'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' 
function='0x0'/>
+    </memballoon>
+  </devices>
+</domain>
diff --git a/tests/qemuxml2xmloutdata/virtio-9p-multidevs.x86_64-latest.xml 
b/tests/qemuxml2xmloutdata/virtio-9p-multidevs.x86_64-latest.xml
new file mode 100644
index 0000000000..4ea75ff8e3
--- /dev/null
+++ b/tests/qemuxml2xmloutdata/virtio-9p-multidevs.x86_64-latest.xml
@@ -0,0 +1,56 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219136</memory>
+  <currentMemory unit='KiB'>219136</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <cpu mode='custom' match='exact' check='none'>
+    <model fallback='forbid'>qemu64</model>
+  </cpu>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <controller type='usb' index='0' model='piix3-uhci'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' 
function='0x2'/>
+    </controller>
+    <controller type='ide' index='0'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' 
function='0x1'/>
+    </controller>
+    <controller type='pci' index='0' model='pci-root'/>
+    <filesystem type='mount' accessmode='mapped' multidevs='remap'>
+      <source dir='/export/fs0'/>
+      <target dir='fs0'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' 
function='0x0'/>
+    </filesystem>
+    <filesystem type='mount' accessmode='mapped' multidevs='forbid'>
+      <source dir='/export/fs1'/>
+      <target dir='fs1'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' 
function='0x0'/>
+    </filesystem>
+    <filesystem type='mount' accessmode='mapped' multidevs='warn'>
+      <source dir='/export/fs2'/>
+      <target dir='fs2'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' 
function='0x0'/>
+    </filesystem>
+    <serial type='pty'>
+      <target type='isa-serial' port='0'>
+        <model name='isa-serial'/>
+      </target>
+    </serial>
+    <console type='pty'>
+      <target type='serial' port='0'/>
+    </console>
+    <input type='mouse' bus='ps2'/>
+    <input type='keyboard' bus='ps2'/>
+    <memballoon model='virtio'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' 
function='0x0'/>
+    </memballoon>
+  </devices>
+</domain>
diff --git a/tests/qemuxml2xmltest.c b/tests/qemuxml2xmltest.c
index a0d27fdcf8..b4c83fccca 100644
--- a/tests/qemuxml2xmltest.c
+++ b/tests/qemuxml2xmltest.c
@@ -1485,6 +1485,8 @@ mymain(void)
     DO_TEST_CAPS_ARCH_LATEST("x86_64-default-cpu-kvm-q35-4.2", "x86_64");
     DO_TEST_CAPS_ARCH_LATEST("x86_64-default-cpu-tcg-q35-4.2", "x86_64");
 
+    DO_TEST_CAPS_LATEST("virtio-9p-multidevs");
+
     if (getenv("LIBVIRT_SKIP_CLEANUP") == NULL)
         virFileDeleteTree(fakerootdir);
 
-- 
2.20.1


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