Re: [PATCH v2] vhost: don't set vring call if guest notifiers is not enabled

[Gavin Liu]

Hi Michael,

――――――――――――――――――――――――――――――――――――― ✀  ―――――――――――――――――――――――――――――――――――――
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) ――
___________________________________________________________________________________________________________________________


When I analyze the problem, the scenario might be like this：
一、vhost user server：
   1：set call fd -1
   2：The function vhost_user_set_vring_call invokes vhost_set_vring_file to 
send the file descriptor information, where fd=-1, thus fd_num=0
   3: The function call vhost_user_write(dev, &msg, fds, fd_num) sends to the 
vhost user client.  
   
二、vhost user client
   1: The function call vhost_user_read receives the file descriptor 
information sent by the server.
   2: SocketChardev *s = SOCKET_CHARDEV(chr); Update s->read_msgfds_num = 
msgfds_num = 0.
   
三、tests/qtest/vhost-user-test
   1：vhost-user-test invokes the chr_read function to handle the messages sent 
by the server.
   2：The chr_read function initializes the local variable fd to -1.
   3：
        case VHOST_USER_SET_VRING_CALL:
                        qemu_chr_fe_get_msgfds(chr, &fd, 1); 
                        
!!! Because s->read_msgfds_num = 0, after the function 
qemu_chr_fe_get_msgfds(chr, &fd, 1) is executed, fd=-1.!!!

                        g_unix_set_fd_nonblocking(fd, true, &err); 
                        
!!! An exception will occur when setting nonblocking when fd < 0. !!!   
                        g_assert_no_error(err);
                                                
四、Offer a modification suggestion
   1：Refer to the tcp_chr_recv function:
                                                static ssize_t 
tcp_chr_recv(Chardev *chr, char *buf, size_t len)
                                                {
                                                        SocketChardev *s = 
SOCKET_CHARDEV(chr);
                                                          -----
                                                          ----
                                                        for (i = 0; i < 
s->read_msgfds_num; i++) {
                                                                int fd = 
s->read_msgfds[i];
                                                                if (fd < 0) {
                                !!!!When fd < 0, the qemu_socket_set_block 
function will not be called !!!!                     
                                                                        
continue;
                                                                }

                                                                /* O_NONBLOCK 
is preserved across SCM_RIGHTS so reset it */
                                                                
qemu_socket_set_block(fd);
                                
   2: modify chr_read function 

                                case VHOST_USER_SET_VRING_CALL:
                                                qemu_chr_fe_get_msgfds(chr, 
&fd, 1); 
                                                if (fd <0)   /* Add a condition 
check. */
                                                        break;
                                                g_unix_set_fd_nonblocking(fd, 
true, &err); 
                                                g_assert_no_error(err);

Best regards,
Yuxue Liu


__________________________________________________________________________________________________

----- Original Message -----
From: Michael S. Tsirkin mst@redhat.com
Sent: April 9, 2024 17:31
To: Gavin Liu gavin.liu@jaguarmicro.com
Cc: jasowang@redhat.com; qemu-stable@nongnu.org; qemu-devel@nongnu.org
Subject: Re: [PATCH] vhost: don't set vring call if no enabled msix



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On Mon, Apr 08, 2024 at 03:33:11PM +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 commit 96a3d98d2cdbd897ff5ab33427aa4cfb94077665, the author 
> provided a good solution. However, because the guest OS's driver(e.g., 
> virtio-net pmd) may not enable the msix capability, the function 
> k->query_guest_notifiers(qbus->parent) may return false, resulting in 
> the expected effect not being achieved. To address this issue, modify 
> the conditional statement.
>
> Signed-off-by: Yuxue Liu <yuxue.liu@jaguarmicro.com>


I tested v1 and it fails. Sent as reply on that patch.
Since all you did is tweak description and title the problem is probably still 
there in v2.

> ---
> V2: Update commit description and title
>
>  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