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Tsirkin" To: Gavin Shan Cc: Will Deacon , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, jasowang@redhat.com, xuanzhuo@linux.alibaba.com, yihyu@redhat.com, shan.gavin@gmail.com, linux-arm-kernel@lists.infradead.org, Catalin Marinas , mochs@nvidia.com Subject: Re: [PATCH] virtio_ring: Fix the stale index in available ring Message-ID: <20240319020949-mutt-send-email-mst@kernel.org> References: <20240314074923.426688-1-gshan@redhat.com> <20240318165924.GA1824@willie-the-truck> <35a6bcef-27cf-4626-a41d-9ec0a338fe28@redhat.com> <20240319020905-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20240319020905-mutt-send-email-mst@kernel.org> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Mar 19, 2024 at 02:09:34AM -0400, Michael S. Tsirkin wrote: > On Tue, Mar 19, 2024 at 02:59:23PM +1000, Gavin Shan wrote: > > On 3/19/24 02:59, Will Deacon wrote: > > > On Thu, Mar 14, 2024 at 05:49:23PM +1000, Gavin Shan wrote: > > > > The issue is reported by Yihuang Yu who have 'netperf' test on > > > > NVidia's grace-grace and grace-hopper machines. The 'netperf' > > > > client is started in the VM hosted by grace-hopper machine, > > > > while the 'netperf' server is running on grace-grace machine. > > > > > > > > The VM is started with virtio-net and vhost has been enabled. > > > > We observe a error message spew from VM and then soft-lockup > > > > report. The error message indicates the data associated with > > > > the descriptor (index: 135) has been released, and the queue > > > > is marked as broken. It eventually leads to the endless effort > > > > to fetch free buffer (skb) in drivers/net/virtio_net.c::start_xmit() > > > > and soft-lockup. The stale index 135 is fetched from the available > > > > ring and published to the used ring by vhost, meaning we have > > > > disordred write to the available ring element and available index. > > > > > > > > /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \ > > > > -accel kvm -machine virt,gic-version=host \ > > > > : \ > > > > -netdev tap,id=vnet0,vhost=on \ > > > > -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0 \ > > > > > > > > [ 19.993158] virtio_net virtio1: output.0:id 135 is not a head! > > > > > > > > Fix the issue by replacing virtio_wmb(vq->weak_barriers) with stronger > > > > virtio_mb(false), equivalent to replaced 'dmb' by 'dsb' instruction on > > > > ARM64. It should work for other architectures, but performance loss is > > > > expected. > > > > > > > > Cc: stable@vger.kernel.org > > > > Reported-by: Yihuang Yu > > > > Signed-off-by: Gavin Shan > > > > --- > > > > drivers/virtio/virtio_ring.c | 12 +++++++++--- > > > > 1 file changed, 9 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > index 49299b1f9ec7..7d852811c912 100644 > > > > --- a/drivers/virtio/virtio_ring.c > > > > +++ b/drivers/virtio/virtio_ring.c > > > > @@ -687,9 +687,15 @@ static inline int virtqueue_add_split(struct virtqueue *_vq, > > > > avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1); > > > > vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head); > > > > - /* Descriptors and available array need to be set before we expose the > > > > - * new available array entries. */ > > > > - virtio_wmb(vq->weak_barriers); > > > > + /* > > > > + * Descriptors and available array need to be set before we expose > > > > + * the new available array entries. virtio_wmb() should be enough > > > > + * to ensuere the order theoretically. However, a stronger barrier > > > > + * is needed by ARM64. Otherwise, the stale data can be observed > > > > + * by the host (vhost). A stronger barrier should work for other > > > > + * architectures, but performance loss is expected. > > > > + */ > > > > + virtio_mb(false); > > > > vq->split.avail_idx_shadow++; > > > > vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, > > > > vq->split.avail_idx_shadow); > > > > > > Replacing a DMB with a DSB is _very_ unlikely to be the correct solution > > > here, especially when ordering accesses to coherent memory. > > > > > > In practice, either the larger timing different from the DSB or the fact > > > that you're going from a Store->Store barrier to a full barrier is what > > > makes things "work" for you. Have you tried, for example, a DMB SY > > > (e.g. via __smb_mb()). > > > > > > We definitely shouldn't take changes like this without a proper > > > explanation of what is going on. > > > > > > > Thanks for your comments, Will. > > > > Yes, DMB should work for us. However, it seems this instruction has issues on > > NVidia's grace-hopper. It's hard for me to understand how DMB and DSB works > > from hardware level. I agree it's not the solution to replace DMB with DSB > > before we fully understand the root cause. > > > > I tried the possible replacement like below. __smp_mb() can avoid the issue like > > __mb() does. __ndelay(10) can avoid the issue, but __ndelay(9) doesn't. > > > > static inline int virtqueue_add_split(struct virtqueue *_vq, ...) > > { > > : > > /* Put entry in available array (but don't update avail->idx until they > > * do sync). */ > > avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1); > > vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head); > > > > /* Descriptors and available array need to be set before we expose the > > * new available array entries. */ > > // Broken: virtio_wmb(vq->weak_barriers); > > // Broken: __dma_mb(); > > // Work: __mb(); > > // Work: __smp_mb(); Did you try __smp_wmb ? And wmb? > > // Work: __ndelay(100); > > // Work: __ndelay(10); > > // Broken: __ndelay(9); > > > > vq->split.avail_idx_shadow++; > > vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, > > vq->split.avail_idx_shadow); > > What if you stick __ndelay here? And keep virtio_wmb above? > > > vq->num_added++; > > > > pr_debug("Added buffer head %i to %p\n", head, vq); > > END_USE(vq); > > : > > } > > > > I also tried to measure the consumed time for various barrier-relative instructions using > > ktime_get_ns() which should have consumed most of the time. __smb_mb() is slower than > > __smp_wmb() but faster than __mb() > > > > Instruction Range of used time in ns > > ---------------------------------------------- > > __smp_wmb() [32 1128032] > > __smp_mb() [32 1160096] > > __mb() [32 1162496] > > > > Thanks, > > Gavin