[RFC] qemu: virtiofs can be used without NUMA nodes

[Marc Hartmayer <mhartmay@xxxxxxxxxxxxx>]

...if a machine memory-backend using shared memory is configured for
the guest. This is especially important for QEMU machine types that
don't have NUMA but virtiofs support.

An example snippet:

  <domain type='kvm'>
    <name>test</name>
    <memory unit='KiB'>2097152</memory>
    <memoryBacking>
      <access mode='shared'/>
    </memoryBacking>
    <devices>
      <filesystem type='mount' accessmode='passthrough'>
	<driver type='virtiofs'/>
	<source dir='/tmp/test'/>
	<target dir='coffee'/>
      </filesystem>
      ...
    </devices>
    ...
  </domain>

and the corresponding QEMU command line:

  /usr/bin/qemu-system-s390x \
  -machine s390-ccw-virtio-5.2,memory-backend=s390.ram \
  -m 2048 \
  -object
  memory-backend-file,id=s390.ram,mem-path=/var/lib/libvirt/qemu/ram/46-test/s390.ram,share=yes,size=2147483648 \
  ...

Signed-off-by: Marc Hartmayer <mhartmay@xxxxxxxxxxxxx>
---
Note: There are still some TODOs left... e.g. adapt the virtiofs
documentation of libvirt.
---
 src/qemu/qemu_validate.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_validate.c b/src/qemu/qemu_validate.c
index a212605579d2..077a85b30802 100644
--- a/src/qemu/qemu_validate.c
+++ b/src/qemu/qemu_validate.c
@@ -3470,14 +3470,21 @@ qemuValidateDomainDeviceDefGraphics(const virDomainGraphicsDef *graphics,
 
 
 static int
-qemuValidateDomainDefVirtioFSSharedMemory(const virDomainDef *def)
+qemuValidateDomainDefVirtioFSSharedMemory(const virDomainDef *def,
+                                          virQEMUCapsPtr qemuCaps)
 {
+    const char *defaultRAMId = virQEMUCapsGetMachineDefaultRAMid(qemuCaps,
+                                                                 def->virtType,
+                                                                 def->os.machine);
     size_t numa_nodes = virDomainNumaGetNodeCount(def->numa);
     size_t i;
 
-    if (numa_nodes == 0) {
+    if (numa_nodes == 0 &&
+        !(defaultRAMId && def->mem.access == VIR_DOMAIN_MEMORY_ACCESS_SHARED)) {
+        /* TODO do we need further checks here (e.g. check whether
+         * memory backend is supported by the QEMU binary)? */
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
-                       _("virtiofs requires one or more NUMA nodes"));
+                       _("virtiofs requires shared memory"));
         return -1;
     }
 
@@ -3591,7 +3598,7 @@ qemuValidateDomainDeviceDefFS(virDomainFSDefPtr fs,
                            _("virtiofs does not support multidevs"));
             return -1;
         }
-        if (qemuValidateDomainDefVirtioFSSharedMemory(def) < 0)
+        if (qemuValidateDomainDefVirtioFSSharedMemory(def, qemuCaps) < 0)
             return -1;
         break;
 
-- 
2.25.4