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Tsirkin" To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, virtualization@lists.linux.dev, kvm@vger.kernel.org, Chandra Merla , Stable@vger.kernel.org, Cornelia Huck , Thomas Huth , Halil Pasic , Eric Farman , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Wei Wang Subject: Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues Message-ID: <20250403085637-mutt-send-email-mst@kernel.org> References: <20250402203621.940090-1-david@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250402203621.940090-1-david@redhat.com> On Wed, Apr 02, 2025 at 10:36:21PM +0200, David Hildenbrand wrote: > If we finds a vq without a name in our input array in > virtio_ccw_find_vqs(), we treat it as "non-existing" and set the vq pointer > to NULL; we will not call virtio_ccw_setup_vq() to allocate/setup a vq. > > Consequently, we create only a queue if it actually exists (name != NULL) > and assign an incremental queue index to each such existing queue. > > However, in virtio_ccw_register_adapter_ind()->get_airq_indicator() we > will not ignore these "non-existing queues", but instead assign an airq > indicator to them. > > Besides never releasing them in virtio_ccw_drop_indicators() (because > there is no virtqueue), the bigger issue seems to be that there will be a > disagreement between the device and the Linux guest about the airq > indicator to be used for notifying a queue, because the indicator bit > for adapter I/O interrupt is derived from the queue index. > > The virtio spec states under "Setting Up Two-Stage Queue Indicators": > > ... indicator contains the guest address of an area wherein the > indicators for the devices are contained, starting at bit_nr, one > bit per virtqueue of the device. > > And further in "Notification via Adapter I/O Interrupts": > > For notifying the driver of virtqueue buffers, the device sets the > bit in the guest-provided indicator area at the corresponding > offset. > > For example, QEMU uses in virtio_ccw_notify() the queue index (passed as > "vector") to select the relevant indicator bit. If a queue does not exist, > it does not have a corresponding indicator bit assigned, because it > effectively doesn't have a queue index. > > Using a virtio-balloon-ccw device under QEMU with free-page-hinting > disabled ("free-page-hint=off") but free-page-reporting enabled > ("free-page-reporting=on") will result in free page reporting > not working as expected: in the virtio_balloon driver, we'll be stuck > forever in virtballoon_free_page_report()->wait_event(), because the > waitqueue will not be woken up as the notification from the device is > lost: it would use the wrong indicator bit. > > Free page reporting stops working and we get splats (when configured to > detect hung wqs) like: > > INFO: task kworker/1:3:463 blocked for more than 61 seconds. > Not tainted 6.14.0 #4 > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > task:kworker/1:3 [...] > Workqueue: events page_reporting_process > Call Trace: > [<000002f404e6dfb2>] __schedule+0x402/0x1640 > [<000002f404e6f22e>] schedule+0x3e/0xe0 > [<000002f3846a88fa>] virtballoon_free_page_report+0xaa/0x110 [virtio_balloon] > [<000002f40435c8a4>] page_reporting_process+0x2e4/0x740 > [<000002f403fd3ee2>] process_one_work+0x1c2/0x400 > [<000002f403fd4b96>] worker_thread+0x296/0x420 > [<000002f403fe10b4>] kthread+0x124/0x290 > [<000002f403f4e0dc>] __ret_from_fork+0x3c/0x60 > [<000002f404e77272>] ret_from_fork+0xa/0x38 > > There was recently a discussion [1] whether the "holes" should be > treated differently again, effectively assigning also non-existing > queues a queue index: that should also fix the issue, but requires other > workarounds to not break existing setups. > > Let's fix it without affecting existing setups for now by properly ignoring > the non-existing queues, so the indicator bits will match the queue > indexes. > > [1] https://lore.kernel.org/all/cover.1720611677.git.mst@redhat.com/ > > Fixes: a229989d975e ("virtio: don't allocate vqs when names[i] = NULL") > Reported-by: Chandra Merla > Cc: > Cc: Cornelia Huck > Cc: Thomas Huth > Cc: Halil Pasic > Cc: Eric Farman > Cc: Heiko Carstens > Cc: Vasily Gorbik > Cc: Alexander Gordeev > Cc: Christian Borntraeger > Cc: Sven Schnelle > Cc: "Michael S. Tsirkin" > Cc: Wei Wang > Signed-off-by: David Hildenbrand Acked-by: Michael S. Tsirkin feel free to merge. > --- > drivers/s390/virtio/virtio_ccw.c | 16 ++++++++++++---- > 1 file changed, 12 insertions(+), 4 deletions(-) > > diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c > index 21fa7ac849e5c..4904b831c0a75 100644 > --- a/drivers/s390/virtio/virtio_ccw.c > +++ b/drivers/s390/virtio/virtio_ccw.c > @@ -302,11 +302,17 @@ static struct airq_info *new_airq_info(int index) > static unsigned long *get_airq_indicator(struct virtqueue *vqs[], int nvqs, > u64 *first, void **airq_info) > { > - int i, j; > + int i, j, queue_idx, highest_queue_idx = -1; > struct airq_info *info; > unsigned long *indicator_addr = NULL; > unsigned long bit, flags; > > + /* Array entries without an actual queue pointer must be ignored. */ > + for (i = 0; i < nvqs; i++) { > + if (vqs[i]) > + highest_queue_idx++; > + } > + > for (i = 0; i < MAX_AIRQ_AREAS && !indicator_addr; i++) { > mutex_lock(&airq_areas_lock); > if (!airq_areas[i]) > @@ -316,7 +322,7 @@ static unsigned long *get_airq_indicator(struct virtqueue *vqs[], int nvqs, > if (!info) > return NULL; > write_lock_irqsave(&info->lock, flags); > - bit = airq_iv_alloc(info->aiv, nvqs); > + bit = airq_iv_alloc(info->aiv, highest_queue_idx + 1); > if (bit == -1UL) { > /* Not enough vacancies. */ > write_unlock_irqrestore(&info->lock, flags); > @@ -325,8 +331,10 @@ static unsigned long *get_airq_indicator(struct virtqueue *vqs[], int nvqs, > *first = bit; > *airq_info = info; > indicator_addr = info->aiv->vector; > - for (j = 0; j < nvqs; j++) { > - airq_iv_set_ptr(info->aiv, bit + j, > + for (j = 0, queue_idx = 0; j < nvqs; j++) { > + if (!vqs[j]) > + continue; > + airq_iv_set_ptr(info->aiv, bit + queue_idx++, > (unsigned long)vqs[j]); > } > write_unlock_irqrestore(&info->lock, flags); > -- > 2.48.1