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Wed, 27 Mar 2024 17:27:39 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGvpTosFo7/pLx/mYgr8vyc9usrDD6UtWd6JJ1sl7V79VUxz2b37iP92LXtAbN+rvgAGGkAwA== X-Received: by 2002:a05:6a00:4b16:b0:6e6:8b59:1bad with SMTP id kq22-20020a056a004b1600b006e68b591badmr1548160pfb.25.1711585659160; Wed, 27 Mar 2024 17:27:39 -0700 (PDT) Received: from [192.168.68.51] ([43.252.115.31]) by smtp.gmail.com with ESMTPSA id b7-20020a62cf07000000b006eac433b2casm146099pfg.128.2024.03.27.17.27.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 Mar 2024 17:27:37 -0700 (PDT) Message-ID: <48b1cc13-849f-4f36-b328-175417a1e4d2@redhat.com> Date: Thu, 28 Mar 2024 10:27:33 +1000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty() Content-Language: en-US To: Jason Wang Cc: virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, mst@redhat.com, davem@davemloft.net, stefanha@redhat.com, sgarzare@redhat.com, keirf@google.com, yihyu@redhat.com, shan.gavin@gmail.com, Will Deacon References: <20240326233846.1086253-1-gshan@redhat.com> <20240326233846.1086253-2-gshan@redhat.com> <92ac171a-0004-45ef-a6c8-01b1c135a8ca@redhat.com> From: Gavin Shan In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 3/27/24 17:42, Jason Wang wrote: > On Wed, Mar 27, 2024 at 3:35 PM Gavin Shan wrote: >> >> On 3/27/24 14:08, Gavin Shan wrote: >>> On 3/27/24 12:44, Jason Wang wrote: >>>> On Wed, Mar 27, 2024 at 10:34 AM Jason Wang wrote: >>>>> On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan wrote: >>>>>> >>>>>> A smp_rmb() has been missed in vhost_vq_avail_empty(), spotted by >>>>>> Will Deacon . Otherwise, it's not ensured the >>>>>> available ring entries pushed by guest can be observed by vhost >>>>>> in time, leading to stale available ring entries fetched by vhost >>>>>> in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's >>>>>> grace-hopper (ARM64) platform. >>>>>> >>>>>> /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \ >>>>>> -accel kvm -machine virt,gic-version=host -cpu host \ >>>>>> -smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \ >>>>>> -m 4096M,slots=16,maxmem=64G \ >>>>>> -object memory-backend-ram,id=mem0,size=4096M \ >>>>>> : \ >>>>>> -netdev tap,id=vnet0,vhost=true \ >>>>>> -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0 >>>>>> : >>>>>> guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM >>>>>> virtio_net virtio0: output.0:id 100 is not a head! >>>>>> >>>>>> Add the missed smp_rmb() in vhost_vq_avail_empty(). Note that it >>>>>> should be safe until vq->avail_idx is changed by commit 275bf960ac697 >>>>>> ("vhost: better detection of available buffers"). >>>>>> >>>>>> Fixes: 275bf960ac697 ("vhost: better detection of available buffers") >>>>>> Cc: # v4.11+ >>>>>> Reported-by: Yihuang Yu >>>>>> Signed-off-by: Gavin Shan >>>>>> --- >>>>>> drivers/vhost/vhost.c | 11 ++++++++++- >>>>>> 1 file changed, 10 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >>>>>> index 045f666b4f12..00445ab172b3 100644 >>>>>> --- a/drivers/vhost/vhost.c >>>>>> +++ b/drivers/vhost/vhost.c >>>>>> @@ -2799,9 +2799,18 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq) >>>>>> r = vhost_get_avail_idx(vq, &avail_idx); >>>>>> if (unlikely(r)) >>>>>> return false; >>>>>> + >>>>>> vq->avail_idx = vhost16_to_cpu(vq, avail_idx); >>>>>> + if (vq->avail_idx != vq->last_avail_idx) { >>>>>> + /* Similar to what's done in vhost_get_vq_desc(), we need >>>>>> + * to ensure the available ring entries have been exposed >>>>>> + * by guest. >>>>>> + */ >>>>> >>>>> We need to be more verbose here. For example, which load needs to be >>>>> ordered with which load. >>>>> >>>>> The rmb in vhost_get_vq_desc() is used to order the load of avail idx >>>>> and the load of head. It is paired with e.g virtio_wmb() in >>>>> virtqueue_add_split(). >>>>> >>>>> vhost_vq_avail_empty() are mostly used as a hint in >>>>> vhost_net_busy_poll() which is under the protection of the vq mutex. >>>>> >>>>> An exception is the tx_can_batch(), but in that case it doesn't even >>>>> want to read the head. >>>> >>>> Ok, if it is needed only in that path, maybe we can move the barriers there. >>>> >>> >>> [cc Will Deacon] >>> >>> Jason, appreciate for your review and comments. I think PATCH[1/2] is >>> the fix for the hypothesis, meaning PATCH[2/2] is the real fix. However, >>> it would be nice to fix all of them in one shoot. I will try with PATCH[2/2] >>> only to see if our issue will disappear or not. However, the issue still >>> exists if PATCH[2/2] is missed. >>> >> >> Jason, PATCH[2/2] is sufficient to fix our current issue. I tried with PATCH[2/2] >> only and unable to hit the issue. However, PATCH[1/2] may be needed by other scenarios. >> So it would be nice to fix them in one shoot. > > Yes, see below. > >> >> >>> Firstly, We were failing on the transmit queue and {tvq, rvq}->busyloop_timeout >>> == false if I remember correctly. So the added smp_rmb() in vhost_vq_avail_empty() >>> is only a concern to tx_can_batch(). A mutex isn't enough to ensure the order >>> for the available index and available ring entry (head). For example, vhost_vq_avail_empty() >>> called by tx_can_batch() can see next available index, but its corresponding >>> available ring entry (head) may not be seen by vhost yet if smp_rmb() is missed. >>> The next call to get_tx_bufs(), where the available ring entry (head) doesn't >>> arrived yet, leading to stale available ring entry (head) being fetched. >>> >>> handle_tx_copy >>> get_tx_bufs // smp_rmb() won't be executed when vq->avail_idx != vq->last_avail_idx >>> tx_can_batch >>> vhost_vq_avail_empty // vq->avail_idx is updated from vq->avail->idx >>> >>> The reason why I added smp_rmb() to vhost_vq_avail_empty() is because the function >>> is a exposed API, even it's only used by drivers/vhost/net.c at present. It means >>> the API has been broken internally. So it seems more appropriate to fix it up in >>> vhost_vq_avail_empty() so that the API's users needn't worry about the memory access >>> order. > > When tx_can_batch returns true it means there's still pending tx > buffers. Since it might read indices so it still can bypass the > smp_rmb() in the vhost_get_vq_desc(). > > I'd suggest adding those above to change log. > > With this, > > Acked-by: Jason Wang > Thanks, Jason. The change log has been improved as you suggested in v3, which was posted for further review. Thanks, Gavin >>> >>>>> >>>>> >>>>>> + smp_rmb(); >>>>>> + return false; >>>>>> + } >>>>>> >>>>>> - return vq->avail_idx == vq->last_avail_idx; >>>>>> + return true; >>>>>> } >>>>>> EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);