Re: [PATCH] virtio: Always reset vhost devices

[Hanna Czenczek <hreitz@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 2948 bytes --]

On 10.07.24 18:28, Stefan Hajnoczi wrote:
> On Wed, 10 Jul 2024 at 13:25, Hanna Czenczek<hreitz@redhat.com>  wrote:
>> Requiring `vhost_started` to be true for resetting vhost devices in
>> `virtio_reset()` seems like the wrong condition: Most importantly, the
>> preceding `virtio_set_status(vdev, 0)` call will (for vhost devices) end
>> up in `vhost_dev_stop()` (through vhost devices' `.set_status`
>> implementations), setting `vdev->vhost_started = false`.  Therefore, the
>> gated `vhost_reset_device()` call is unreachable.
>>
>> `vhost_started` is not documented, so it is hard to say what exactly it
>> is supposed to mean, but judging from the fact that `vhost_dev_start()`
>> sets it and `vhost_dev_stop()` clears it, it seems like it indicates
>> whether there is a vhost back-end, and whether that back-end is
>> currently running and processing virtio requests.
>>
>> Making a reset conditional on whether the vhost back-end is processing
>> virtio requests seems wrong; in fact, it is probably better to reset it
>> only when it is not currently processing requests, which is exactly the
>> current order of operations in `virtio_reset()`: First, the back-end is
>> stopped through `virtio_set_status(vdev, 0)`, then we want to send a
>> reset.
>>
>> Therefore, we should drop the `vhost_started` condition, but in its
>> stead we then have to verify that we can indeed send a reset to this
>> vhost device, by not just checking `k->get_vhost != NULL` (introduced by
>> commit 95e1019a4a9), but also that the vhost back-end is connected
>> (`hdev = k->get_vhost(); hdev != NULL && hdev->vhost_ops != NULL`).
>>
>> Signed-off-by: Hanna Czenczek<hreitz@redhat.com>
> I think an additional SET_STATUS 0 call is made to the vDPA vhost
> backend after this patch, but that seems fine.
>
> Reviewed-by: Stefan Hajnoczi<stefanha@redhat.com>

Thanks!  I agree that double-sending SET_STATUS with the same value 
should be fine.The virtio specification states: “The device status field 
starts out as 0, and is reinitialized to 0 by the device during reset.” 
– I interpret that to mean that (re-)setting the field to 0 is always 
OK. Hanna
>> ---
>>   hw/virtio/virtio.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
>> index 893a072c9d..4410d62126 100644
>> --- a/hw/virtio/virtio.c
>> +++ b/hw/virtio/virtio.c
>> @@ -2146,8 +2146,12 @@ void virtio_reset(void *opaque)
>>           vdev->device_endian = virtio_default_endian();
>>       }
>>
>> -    if (vdev->vhost_started && k->get_vhost) {
>> -        vhost_reset_device(k->get_vhost(vdev));
>> +    if (k->get_vhost) {
>> +        struct vhost_dev *hdev = k->get_vhost(vdev);
>> +        /* Only reset when vhost back-end is connected */
>> +        if (hdev && hdev->vhost_ops) {
>> +            vhost_reset_device(hdev);
>> +        }
>>       }
>>
>>       if (k->reset) {
>> --
>> 2.45.2
>>
>>

[-- Attachment #2: Type: text/html, Size: 3932 bytes --]