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

[Michal Privoznik]

On 10/6/20 6:20 PM, Marc Hartmayer wrote:
> ...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 at linux.ibm.com>
> ---
> Note: There are still some TODOs left... e.g. adapt the virtiofs
> documentation of libvirt.

Yep, but looks good.

> ---
>   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)? */

I don't think we need that. memory backends can't be compiled out (well, 
unless a distro has a patch on the top of qemu which would do exactly 
that), can defaultRAMId exposed is strictly newer than memory backends.

I think this comment can be removed and the rest can be kept as is (plus 
  the docs).

Michal