[libvirt] [PATCH v2 5/5] qemu: Enable memory-backend-file.discard-data whenever possible

[Eduardo Habkost]

On Tue, Apr 17, 2018 at 01:16:42PM +0200, Michal Privoznik wrote:
> https://bugzilla.redhat.com/show_bug.cgi?id=1480668
> 
> The cases when we cannot enable this optimization are:
>   1) nvdimms
>   2) if memAccess='shared'

The specific use case for discard-data=on uses share=on, see:
https://bugzilla.redhat.com/show_bug.cgi?id=1460848#c4

It looks like this will require a explicit XML element, after
all.


> 
> Otherwise it is safe to enable it.
> 
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/qemu/qemu_command.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> index 527a35779d..b920f5c3e4 100644
> --- a/src/qemu/qemu_command.c
> +++ b/src/qemu/qemu_command.c
> @@ -3121,6 +3121,13 @@ qemuBuildMemoryBackendStr(virJSONValuePtr *backendProps,
>                                    NULL) < 0)
>              goto cleanup;
>  
> +        if (!mem->nvdimmPath && memAccess != VIR_DOMAIN_MEMORY_ACCESS_SHARED &&
> +            virQEMUCapsGet(qemuCaps, QEMU_CAPS_OBJECT_MEMORY_FILE_DISCARD) &&
> +            virJSONValueObjectAdd(props,
> +                                  "B:discard-data", true,
> +                                  NULL) < 0)
> +            goto cleanup;
> +
>          switch (memAccess) {
>          case VIR_DOMAIN_MEMORY_ACCESS_SHARED:
>              if (virJSONValueObjectAdd(props, "b:share", true, NULL) < 0)
> -- 
> 2.16.1
> 

-- 
Eduardo