[PATCH] conf: check vhost-user queues with vcpus

[Martin Kletzander]

The ping for this patch got threaded weirdly, so I just stumbled upon
this now.

The subject could use s/with/against/ or could be something like "Limit
the number of vhost-user queues to the number of vcpus", but a question
below.

On Thu, Dec 08, 2022 at 08:52:19PM +0800, Jiang Jiacheng wrote:
>With kernel without the ref patch, if queues > vcpus, interrupts
>will be centralized on one vcpu affecting guest performance. After
>the ref patch merged, the queues whose number is greater than the
>number of vcpus will not be used.
>
>Considering the above, it's better to check the counts of vhost-user
>queues and vcpus.
>
>ref:
>https://patchwork.kernel.org/project/linux-scsi/cover/1553682995-5682-1-git-send-email-dongli.zhang@oracle.com/
>

It makes sense, although even from the kernel patch I am not sure
whether this should be checked against maxvcpus (i.e. using
virDomainDefGetVcpusMax()) or whether this is more related to the number
of physical CPUs on the host.  Definitely not the number of online guest
vcpus.

>Signed-off-by: Jiang Jiacheng <jiangjiacheng at huawei.com>
>---
> src/conf/domain_validate.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
>diff --git a/src/conf/domain_validate.c b/src/conf/domain_validate.c
>index 95b8d9b419..6106e79999 100644
>--- a/src/conf/domain_validate.c
>+++ b/src/conf/domain_validate.c
>@@ -308,7 +308,7 @@ virSecurityDeviceLabelDefValidate(virSecurityDeviceLabelDef **seclabels,
>
>
> static int
>-virDomainDiskVhostUserValidate(const virDomainDiskDef *disk)
>+virDomainDiskVhostUserValidate(const virDomainDef *def, const virDomainDiskDef *disk)
> {
>     if (disk->bus != VIR_DOMAIN_DISK_BUS_VIRTIO) {
>         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
>@@ -465,6 +465,12 @@ virDomainDiskVhostUserValidate(const virDomainDiskDef *disk)
>         return -1;
>     }
>
>+    if (disk->queues > virDomainDefGetVcpus(def)) {
>+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
>+                       _("vhost-user disk queues must <= vcpus"));
>+        return -1;
>+    }
>+
>     return 0;
> }
>
>@@ -807,7 +813,7 @@ virDomainDiskDefValidate(const virDomainDef *def,
>     }
>
>     if (disk->src->type == VIR_STORAGE_TYPE_VHOST_USER &&
>-        virDomainDiskVhostUserValidate(disk) < 0) {
>+        virDomainDiskVhostUserValidate(def, disk) < 0) {
>         return -1;
>     }
>
>-- 
>2.33.0
>
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