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[82.53.135.12]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49230a96b47sm56484875e9.13.2026.06.16.07.18.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 16 Jun 2026 07:18:42 -0700 (PDT) Date: Tue, 16 Jun 2026 16:18:26 +0200 From: Stefano Garzarella To: Andrey Drobyshev Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, mst@redhat.com, stefanha@redhat.com, maciej.szmigiero@oracle.com, bchaney@akamai.com, mark.kanda@oracle.com, ptikhomirov@virtuozzo.com, den@openvz.org Subject: Re: [PATCH 3/4] vhost/vsock: suppress EHOSTUNREACH fast-fail during CPR pause Message-ID: References: <20260612165718.433546-1-andrey.drobyshev@virtuozzo.com> <20260612165718.433546-4-andrey.drobyshev@virtuozzo.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20260612165718.433546-4-andrey.drobyshev@virtuozzo.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: TTUo02YnYw40vOKnZzC7zN9Sj-gOXMhZD5rEpg88e7k_1781619524 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit On Fri, Jun 12, 2026 at 07:57:17PM +0300, Andrey Drobyshev wrote: >From: "Denis V. Lunev" > >Earlier commit ("ms/vhost/vsock: Refuse the connection immediately when Please follow https://docs.kernel.org/process/submitting-patches.html#describe-your-changes on how to refer to a commit. >guest isn't ready") added a fast-fail in vhost_transport_send_pkt(). It >rejects every host send with -EHOSTUNREACH until the destination calls >SET_RUNNING(1). The fast-fail condition checks whether device's backends >are dropped, and if they're, the guest is considered to be not ready. Okay, so it's not a regression, I mean without this series that patch is not adding any regression, no? If it's the case, I'll change the wording in the cover letter. > >However, there might be other reasons for backends to be nulled. In >particular, when QEMU is performing CPR (checkpoint-restore) migration, >device ownership is being RESET and SET again, which leads to backends >drop and reattach. If we end up connecting during this window, an >AF_VSOCK client gets -EHOSTUNREACH, which is wrong. Please add this change before starting to support VHOST_RESET_OWNER ioctl in vhost-vsock, otherwise we are breaking the bisectability. > >Add a cpr_paused flag set inside vhost_vsock_drop_backends() when the >backend was previously live, cleared by vhost_vsock_start(). When set, >vhost_transport_send_pkt() queues the skb instead of fast-failing; the >existing kick of send_pkt_work in vhost_vsock_start() drains it on >resume. A device that has never run keeps cpr_paused == false and the >boot-time fast-fail behaviour is preserved. > >Pair the cpr_paused store with the backend store using an >smp_wmb()/smp_rmb() pair so a concurrent sender on a weakly-ordered >architecture never observes (NULL backend, !paused): > >Signed-off-by: Denis V. Lunev >--- > drivers/vhost/vsock.c | 22 +++++++++++++++++++--- > 1 file changed, 19 insertions(+), 3 deletions(-) > >diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c >index e629886e5cf8..bcaba36becd7 100644 >--- a/drivers/vhost/vsock.c >+++ b/drivers/vhost/vsock.c >@@ -61,6 +61,7 @@ struct vhost_vsock { > > u32 guest_cid; > bool seqpacket_allow; >+ bool cpr_paused; /* between stop and next start */ > }; > > static u32 vhost_transport_get_local_cid(void) >@@ -311,11 +312,17 @@ vhost_transport_send_pkt(struct sk_buff *skb, struct net *net) > * the mutex would be too expensive in this hot path, and we already have > * all the outcomes covered: if the backend becomes NULL right after the check, > * vhost_transport_do_send_pkt() will check it under the mutex anyway. >+ * >+ * Don't fast-fail if cpr_paused is set, keep queueing skbs instead. >+ * The kick in vhost_vsock_start() will drain them on resume. > */ > if (unlikely(!data_race(vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX])))) { >- rcu_read_unlock(); >- kfree_skb(skb); >- return -EHOSTUNREACH; >+ smp_rmb(); /* pairs with smp_wmb() in start/drop_backends */ >+ if (!READ_ONCE(vsock->cpr_paused)) { Can we avoid this which is not really readable and maybe add a single variable to control the fast-fail at all? I mean replacing both cpr_paused + backend-pointer with a single `started` flag: set it to false at open, true on start via smp_store_release(), back to false on normal stop, and leave it true during CPR pause. The reader in send_pkt can do just: if (!smp_load_acquire(&vsock->started)) return -EHOSTUNREACH; WDYT? >+ rcu_read_unlock(); >+ kfree_skb(skb); >+ return -EHOSTUNREACH; >+ } That said claude here is reporting a potential issue that I think we should consider: After VHOST_RESET_OWNER, the guest CID stays in the hash, so vhost_transport_send_pkt() can still find the vsock, skip the fast-fail (cpr_paused=true), and call vhost_vq_work_queue() while vhost_workers_free() is freeing workers without a synchronize_rcu() — risking a use-after-free. Also, any send_pkt_work queued between the last flush and worker teardown gets its VHOST_WORK_QUEUED bit stuck (the vhost task exits without draining), deadlocking host→guest traffic after restart. A synchronize_rcu() in vhost_workers_free() between the rcu_assign_pointer(NULL) loop and the destroy loop would close the use-after-free, and reinitializing send_pkt_work via vhost_work_init() after vhost_dev_reset_owner() returns would clear the stuck QUEUED bit. > } > > if (virtio_vsock_skb_reply(skb)) >@@ -640,6 +647,9 @@ static int vhost_vsock_start(struct vhost_vsock *vsock) > mutex_unlock(&vq->mutex); > } > >+ smp_wmb(); /* pairs with smp_rmb() in send_pkt */ >+ WRITE_ONCE(vsock->cpr_paused, false); >+ > /* Some packets may have been queued before the device was started, > * let's kick the send worker to send them. > */ >@@ -671,6 +681,11 @@ static void vhost_vsock_drop_backends(struct vhost_vsock *vsock) > > lockdep_assert_held(&vsock->dev.mutex); > >+ if (vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX])) { >+ WRITE_ONCE(vsock->cpr_paused, true); >+ smp_wmb(); /* pairs with smp_rmb() in send_pkt */ >+ } Why here and not in vhost_vsock_reset_owner()? Also having this here will set it to true also with VHOST_VSOCK_SET_RUNNING(0), is that right? Thanks, Stefano >+ > for (i = 0; i < ARRAY_SIZE(vsock->vqs); i++) { > vq = &vsock->vqs[i]; > >@@ -728,6 +743,7 @@ static int vhost_vsock_dev_open(struct inode *inode, struct file *file) > > vsock->guest_cid = 0; /* no CID assigned yet */ > vsock->seqpacket_allow = false; >+ vsock->cpr_paused = false; > > atomic_set(&vsock->queued_replies, 0); > >-- >2.47.1 >