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Tsirkin" 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 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> <9b3030d1-cb2c-4ce0-8b24-1074b616fc84@redhat.com> <20240319024025-mutt-send-email-mst@kernel.org> From: Gavin Shan In-Reply-To: <20240319024025-mutt-send-email-mst@kernel.org> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240318_235000_951646_09687FD3 X-CRM114-Status: GOOD ( 21.59 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 3/19/24 16:43, Michael S. Tsirkin wrote: > On Tue, Mar 19, 2024 at 04:38:49PM +1000, Gavin Shan wrote: >> On 3/19/24 16:09, Michael S. Tsirkin wrote: >> >>>>>> 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(); >>>> // 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? >>> >> >> /* 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. */ >> virtio_wmb(vq->weak_barriers); >> vq->split.avail_idx_shadow++; >> vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, >> vq->split.avail_idx_shadow); >> /* Try __ndelay(x) here as Michael suggested >> * >> * Work: __ndelay(200); possiblly make it hard to reproduce >> * Broken: __ndelay(100); >> * Broken: __ndelay(20); >> * Broken: __ndelay(10); >> */ >> __ndelay(200); > > So we see that just changing the timing masks the race. > What are you using on the host side? vhost or qemu? > __ndelay(200) may make the issue harder to be reproduce as I understand. More delays here will give vhost relief, reducing the race. The issue is only reproducible when vhost is turned on. Otherwise, we aren't able to hit the issue. -netdev tap,id=vnet0,vhost=true,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown \ -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0 >> >>> >>>> 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel