Re: [PATCH] vhost: don't set vring call if no enabled msix

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 08, 2024 at 02:08:42PM +0800, lyx634449800 wrote:
> When conducting performance testing using testpmd in the guest os,
> it was observed that the performance was lower compared to the
> scenario of direct vfio-pci usage.
> 
> In the virtual machine operating system, even if the virtio device
> does not use msix interrupts, vhost still sets vring call fd. This
> leads to unnecessary performance overhead. If the guest driver does
> not enable msix capability (e.g virtio-net pmd), we should also
> check and clear the vring call fd.
> 
> Signed-off-by: Yuxue Liu <yuxue.liu@jaguarmicro.com>


Fails testing under cross-i686-tci:

https://gitlab.com/mstredhat/qemu/-/jobs/6578881990

36/258 qemu:qtest+qtest-i386 / qtest-i386/ioh3420-test                    OK               0.15s   1 subtests passed
▶  37/258 ERROR:../tests/qtest/qos-test.c:191:subprocess_run_one_test: child process (/aarch64/virt/generic-pcihost/pci-bus-generic/pci-bus/virtio-net-pci/virtio-net/virtio-net-tests/vhost-user/migrate/subprocess [22197]) failed unexpectedly ERROR         
 38/258 qemu:qtest+qtest-i386 / qtest-i386/lpc-ich9-test                   OK               0.16s   1 subtests passed
 37/258 qemu:qtest+qtest-aarch64 / qtest-aarch64/qos-test                  ERROR           13.20s   killed by signal 6 SIGABRT
>>> G_TEST_DBUS_DAEMON=/builds/mstredhat/qemu/tests/dbus-vmstate-daemon.sh PYTHON=/builds/mstredhat/qemu/build/pyvenv/bin/python3 MALLOC_PERTURB_=144 QTEST_QEMU_BINARY=./qemu-system-aarch64 /builds/mstredhat/qemu/build/tests/qtest/qos-test --tap -k
――――――――――――――――――――――――――――――――――――― ✀  ―――――――――――――――――――――――――――――――――――――
stderr:
**
ERROR:../tests/qtest/vhost-user-test.c:468:chr_read: assertion failed (err == NULL): Bad file descriptor (g-unix-error-quark, 0)
**
ERROR:../tests/qtest/qos-test.c:191:subprocess_run_one_test: child process (/aarch64/virt/generic-pcihost/pci-bus-generic/pci-bus/virtio-net-pci/virtio-net/virtio-net-tests/vhost-user/migrate/subprocess [22197]) failed unexpectedly
(test program exited with status code -6)
――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――


> ---
>  hw/virtio/vhost.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index f50180e60e..b972c84e67 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -1266,13 +1266,15 @@ int vhost_virtqueue_start(struct vhost_dev *dev,
>          vhost_virtqueue_mask(dev, vdev, idx, false);
>      }
>  
> -    if (k->query_guest_notifiers &&
> -        k->query_guest_notifiers(qbus->parent) &&
> -        virtio_queue_vector(vdev, idx) == VIRTIO_NO_VECTOR) {
> -        file.fd = -1;
> -        r = dev->vhost_ops->vhost_set_vring_call(dev, &file);
> -        if (r) {
> -            goto fail_vector;
> +    if (k->query_guest_notifiers) {
> +        if (!k->query_guest_notifiers(qbus->parent) ||
> +            (k->query_guest_notifiers(qbus->parent) &&
> +            virtio_queue_vector(vdev, idx) == VIRTIO_NO_VECTOR)) {
> +            file.fd = -1;
> +            r = dev->vhost_ops->vhost_set_vring_call(dev, &file);
> +            if (r) {
> +                goto fail_vector;
> +            }
>          }
>      }
>  
> -- 
> 2.43.0