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(p200300cfd74b1cd51c4c0c09d73bc07d.dip0.t-ipconnect.de. [2003:cf:d74b:1cd5:1c4c:c09:d73b:c07d]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a780a6bc7b4sm251857766b.43.2024.07.11.04.46.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Jul 2024 04:46:34 -0700 (PDT) Content-Type: multipart/alternative; boundary="------------6L37yea10o3eSTCkvkqwK6H0" Message-ID: Date: Thu, 11 Jul 2024 13:46:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] virtio: Always reset vhost devices To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Stefan Hajnoczi , "Michael S . Tsirkin" References: <20240710112310.316551-1-hreitz@redhat.com> Content-Language: en-US From: Hanna Czenczek In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.142, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org This is a multi-part message in MIME format. --------------6L37yea10o3eSTCkvkqwK6H0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10.07.24 18:28, Stefan Hajnoczi wrote: > On Wed, 10 Jul 2024 at 13:25, Hanna Czenczek 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 > 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 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 >> >> --------------6L37yea10o3eSTCkvkqwK6H0 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit
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




    

    

  


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