[libvirt] [PATCH v3 1/5] qemu: caps: add QEMU_CAPS_QCOW2_L2_CACHE_SIZE_CAPPED

[John Ferlan]

On 1/10/19 7:15 AM, Nikolay Shirokovskiy wrote:
> For qemu capable of setting l2-cache-size for qcow2 images
> to INT64_MAX and semantics of upper limit on l2 cache
> size. We can only check this by qemu version (3.1.0) now.
> 
> Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy at virtuozzo.com>
> ---
>  src/qemu/qemu_capabilities.c | 5 +++++
>  src/qemu/qemu_capabilities.h | 1 +
>  2 files changed, 6 insertions(+)
> 

I posted a patch series today that would add/update the capabilities for
3.1 (and 4.0) and thus would add this capability to this patch.

However, given that the only way you've chosen to support/implement this
is via -blockdev, I'm not sure now what the purpose will be unless it's
used to determine whether to use INT64_MAX or some other value.

I see from previous series that it seems to be possible to have:

   -drive xxx,format=qcow2,l2-cache-size=VALUE,...

However, patch3 notes in the commit message that "-drive configuration
is not supported because we can not set l2 cache size down the backing
chain in this case."

So if -blockdev is going to be your the option for support, then I think
it'd be wiser to wait for Peter to finish -blockdev support before
acting on this.

Since -blockdev support allows some qemu version earlier the 3.1 to be
supported, then we revisit this. Additionally, referencing Leonid's
qemu.git commit b749562d98 as the "arbiter" of why INT64_MAX must be
used (since calculating a max max value isn't possible, IIRC).

John

> diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
> index f504db7..e09b13a 100644
> --- a/src/qemu/qemu_capabilities.c
> +++ b/src/qemu/qemu_capabilities.c
> @@ -520,6 +520,7 @@ VIR_ENUM_IMPL(virQEMUCaps, QEMU_CAPS_LAST,
>                /* 325 */
>                "memory-backend-file.pmem",
>                "nvdimm.unarmed",
> +              "qcow2.l2-cache-size.capped",
>      );
>  
>  
> @@ -4256,6 +4257,10 @@ virQEMUCapsInitQMPMonitor(virQEMUCapsPtr qemuCaps,
>          virQEMUCapsSet(qemuCaps, QEMU_CAPS_MACHINE_PSERIES_MAX_CPU_COMPAT);
>      }
>  
> +    /* l2-cache-size before 3001000 does not accept INT64_MAX */
> +    if (qemuCaps->version >= 3001000)
> +        virQEMUCapsSet(qemuCaps, QEMU_CAPS_QCOW2_L2_CACHE_SIZE_CAPPED);
> +
>      if (virQEMUCapsProbeQMPCommands(qemuCaps, mon) < 0)
>          goto cleanup;
>  
> diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
> index 6d5ed8a..e17a1dc 100644
> --- a/src/qemu/qemu_capabilities.h
> +++ b/src/qemu/qemu_capabilities.h
> @@ -504,6 +504,7 @@ typedef enum { /* virQEMUCapsFlags grouping marker for syntax-check */
>      /* 325 */
>      QEMU_CAPS_OBJECT_MEMORY_FILE_PMEM, /* -object memory-backend-file,pmem= */
>      QEMU_CAPS_DEVICE_NVDIMM_UNARMED, /* -device nvdimm,unarmed= */
> +    QEMU_CAPS_QCOW2_L2_CACHE_SIZE_CAPPED, /* -blockdev supports l2-cache-size with INT64_MAX value */
>  
>      QEMU_CAPS_LAST /* this must always be the last item */
>  } virQEMUCapsFlags;
>