From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7A8631ACEDE for ; Tue, 12 May 2026 16:02:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778601732; cv=none; b=NOQG7qGkJDKfIrP0oKe0OO9JLuOsm8HayPChkzouw9k8JG49yJWyTZtQD7NnQTip2nAVac9ujW8Wf+JTUkbB64DcoMe/1ueUi8Zs2KDn21bBz90IFrG9ktKSwXkveQC2TenTo4Q2P1/2ubO31Ii6n+LXHLgs9g5a5sOPO7yUI9o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778601732; c=relaxed/simple; bh=vWr9cfyQCOSfH7IJmIj+myZeDwfb59BivVN4VpsvMRA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=AJVb66GLd3P+ewR9Hzjyhb6zsHzxt7Ej+6blJ6FqI1LltD3lwJ+YS8D/NHPiVKy3YQN5WNEhIq+5YFyliA/JfyzKjOoCVAbaV4C7tdS1FXhEAYi+cCFmr+0lgQvj1PitpZBNgcJyLfhFZbnAdEyKX4MB+8Q5xt9+rwKbXgB1sT8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Y2tRKbwh; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b=atvtpASn; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Y2tRKbwh"; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b="atvtpASn" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1778601729; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=FIUiJKhl00kL6+7DmOinctS+OWNE7gekD3egzsDj+sI=; b=Y2tRKbwhv/iGLQ4ixjTnBtskmyP51cAB2YAu+2IWl0E3wFL8mzGbPQV7mxOxtxfS4iLCYz ON9UPYrXlB57PuKQUi/t73F7eaA14L8TpEE9/KzfvhlvQVtdXSDmz5VXWZ9P7mb/dUOU/0 OypRvEwZSJj18AjmjqrpLSm1slybf4U= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-588-kt-1skqaOLyy6qI5UxuEkw-1; Tue, 12 May 2026 12:02:07 -0400 X-MC-Unique: kt-1skqaOLyy6qI5UxuEkw-1 X-Mimecast-MFC-AGG-ID: kt-1skqaOLyy6qI5UxuEkw_1778601727 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-48e60b55de2so50625585e9.3 for ; Tue, 12 May 2026 09:02:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=google; t=1778601726; x=1779206526; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=FIUiJKhl00kL6+7DmOinctS+OWNE7gekD3egzsDj+sI=; b=atvtpASn5Dfk/7FGVkodl5WTDAnbYKqjdEck4Y0kJDEvRsdXPeLad4OVT9vBDzt2NT a8YgJE6130o3Oh/3iwTa3rLnDyI+m2uOO4ADx2wY9baugbbB01lHy8NhA37ao3tCMZOp PoLyHJIrux79wUq9fuseSjoovEm4bzUVAbB2WdZSvFvHPUe7F4DOQBpp974rk7K0QCZ6 bG1xgip9lKdnIeugkPaP7bzIF9fXyHNqUVMFk1D5SbTJYVwQfOvGpGuuE76JlBDp7cey zbhXdJNhrxs0+rj/fDhtYhw2iRkDG47zHDmsQApID29xIVl80twXXGXEL7ruYRilKkkB 54UQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1778601726; x=1779206526; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=FIUiJKhl00kL6+7DmOinctS+OWNE7gekD3egzsDj+sI=; b=EJxzpGMQ+8ZZoaWXSzdZBZ0cXqlilPE0tkFSlmZjW3nZvAVEV717/88o5Akww0/50Q LabCDnSZRvv2kjbHJ4fjoWePcijdmYGzyGbfpps5qlYcVnkwbMYuPj7KtUHCW7MXxFmJ LAodJpjXL3DrbmS0a/XR2qILzGabHQ6EFddmd0BXPwI1dptDeGakHL1OyeNNRQepfQSc jx1gYDDW4XcUYomPl55q3mTBh8yO8OSP2S/kYaU+AulAB8ig+VCh9nJmRCFloEmunrhF GTgoKIatCk7aMMZoLLoodg+sXkHSzd+OLxF7KfI9xUDSoOfRmbXHdi1jGizneMdcverF Nuog== X-Forwarded-Encrypted: i=1; AFNElJ+b/D0BiVIj/PLgZpTf2fpqMJIuSuYIYSEP+rmIIbGDPJWaR/DUxIeRKOd+pRW7qTE4d70=@vger.kernel.org X-Gm-Message-State: AOJu0Yy5amVcI7zLbVM4cEYdyI8AAGxpbG7/4zNSVPs7uyVMoY8qHwdl VmTpjnZpLu7AXrX+8ogjylgVyVMUgYcSR1abjbs1SZYQ3aZ+pyduMDypPeq7BtuUeRKW6Mag33T I9CODERM918DAdkfKlAYc13WRIZaT6QFukgqz3D0mZf8qDCcauDer0A== X-Gm-Gg: Acq92OHrcE1h2FirFyMU3Fhifqrt+OSql4J9sq/0dBQcf7fQrMBxYIjkMQfVba7v1Pj kX1EBbxBUOBpZb1HRvIF/uDs09BvMfld0YTgRD3AVuGWP6xB7FYXHUJF9iHOOsyu5Aex/ZYC8hm E2GbBj/5197uEuCj9KZO5HfC9cMHlwpRqDIGRa7GYrCJb10k0jUb5RIQ61RUUgFwz8lIXhfLcft jhbWjA+AaMQCcZNedON1BiCdk576aVvtnCAky40EFL38kFxbAhirPyXsj4FFmzj4bGgtEdy/uK3 xkX+2MNjUIE4Twsh16V4f/4wkXPwRHquWNT01D1387AnWE7YKm2QxatUXZRsndbyMFb+onhKpuX VqKu0bRMqUAUt+IcXXUlpBQ0kEnIP8UwGt2wSyCQ/ X-Received: by 2002:a05:600c:444f:b0:488:9439:880d with SMTP id 5b1f17b1804b1-48e8fe84e0bmr59315735e9.29.1778601726357; Tue, 12 May 2026 09:02:06 -0700 (PDT) X-Received: by 2002:a05:600c:444f:b0:488:9439:880d with SMTP id 5b1f17b1804b1-48e8fe84e0bmr59313645e9.29.1778601724684; Tue, 12 May 2026 09:02:04 -0700 (PDT) Received: from redhat.com (IGLD-80-230-48-7.inter.net.il. [80.230.48.7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e8e64385bsm22758065e9.12.2026.05.12.09.02.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 12 May 2026 09:02:03 -0700 (PDT) Date: Tue, 12 May 2026 12:02:00 -0400 From: "Michael S. Tsirkin" To: Stefano Garzarella Cc: Polina Vishneva , "den@openvz.org" , "virtualization@lists.linux.dev" , "stefanha@redhat.com" , "eperezma@redhat.com" , "linux-kernel@vger.kernel.org" , "netdev@vger.kernel.org" , "kvm@vger.kernel.org" , "jasowang@redhat.com" Subject: Re: [PATCH] vhost/vsock: Refuse the connection immediately when guest isn't ready Message-ID: <20260512120019-mutt-send-email-mst@kernel.org> References: <20260511145610.413210-1-polina.vishneva@virtuozzo.com> <962b26d2d1daa9411fb71efab6af2c75d1c5f0d0.camel@virtuozzo.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: On Tue, May 12, 2026 at 05:39:48PM +0200, Stefano Garzarella wrote: > On Tue, May 12, 2026 at 02:32:14PM +0000, Polina Vishneva wrote: > > On Mon, 2026-05-11 at 17:56 +0200, Stefano Garzarella wrote: > > > On Mon, May 11, 2026 at 04:56:10PM +0200, Polina Vishneva wrote: > > > > From: "Denis V. Lunev" > > > > > > > > When the host initiates an AF_VSOCK connect() to a guest that has not > > > > yet loaded the virtio-vsock transport (i.e. still booting), the caller > > > > blocks for VSOCK_DEFAULT_CONNECT_TIMEOUT (2 seconds), because > > > > vhost_transport_do_send_pkt() silently exits when > > > > vhost_vq_get_backend(vq) returns NULL. > > > > > > Can SO_VM_SOCKETS_CONNECT_TIMEOUT helps on this? > > > > It can, but it might be difficult to find a correct timeout. > > > > And, generally, there's no way to distinguish "the guest hasn't yet initialized > > the vq" from "the guest is up and running, but didn't reply to connect() in > > time". That's exactly what this patch is attempting to fix. > > Okay, so please mention this in the commit message, I mean why > SO_VM_SOCKETS_CONNECT_TIMEOUT can't really help. > > > > > > > > > > > > > > If the guest doesn't start listening within this timeout, connect() > > > > returns ETIMEDOUT. > > > > > > > > This delay is usually pointless and it doesn't well align with our > > I still don't understand why this is pointless. If an application wants to > wait while sleeping, it can simply increase the timeout long enough to wait > for the VM to start up and use a single `connect()` call, instead of > continuing to try and wasting CPU cycles unnecessarily. > > Hmm, or maybe not, because the driver will definitely be initialized before > the application that wants to listen on that port, so it will respond that > no one is listening, and the `connect()` call will fail with an `ECONNRESET` > error in any case. Right? > > If it is the case, is the following line in the commit description correct? > > If the guest doesn't start listening within this timeout, connect() > returns ETIMEDOUT. > > I mean, also if the application starts to listen within the timeout, I think > the connect() will fail in any case as I pointed out above (this should be > another point in favour of this change) > > > BTW, I think we should explain this more clearly both here and briefly in > the code as well. > > > > > behavior at other initialization stages: for example, if a connection is > > > > attempted when the guest driver is already loaded, but when nothing is > > > > listening yet, it returns ECONNRESET immediately without any wait. > > > > > > > > Fix this by checking the RX virtqueue backend in > > > > vhost_transport_send_pkt() before queuing. If the backend is NULL, > > > > return -ECONNREFUSED immediately. > > > > > > > > Signed-off-by: Denis V. Lunev > > > > Co-developed-by: Polina Vishneva > > > > Signed-off-by: Polina Vishneva > > > > --- > > > > drivers/vhost/vsock.c | 10 ++++++++++ > > > > 1 file changed, 10 insertions(+) > > > > > > > > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c > > > > index 1d8ec6bed53e..a3f218292c3a 100644 > > > > --- a/drivers/vhost/vsock.c > > > > +++ b/drivers/vhost/vsock.c > > > > @@ -302,6 +302,16 @@ vhost_transport_send_pkt(struct sk_buff *skb, struct net *net) > > > > return -ENODEV; > > > > } > > > > > > > > + /* Fast-fail if the guest hasn't enabled the RX vq yet. Reading > > > > + * private_data without vq->mutex is deliberate: even if the backend becomes > > > > + * NULL right after that check, do_send_pkt() checks it under the mutex. > > > > + */ > > > > + if (!data_race(READ_ONCE(vsock->vqs[VSOCK_VQ_RX].private_data))) > > > > > > Why not using vhost_vq_get_backend() ? > > > > Because it locks the mutex, which is slow and unacceptable in this hot > > path. > > ehm, sorry, which mutex are you talking about? > > I see just a comment about the mutex to be acquired by the caller, but I > don't see any lock there. > > > > > > > > > Also is READ_ONCE() okay without WRITE_ONCE() where it is set ? > > > > It's racy, but as described here in the comment and in the commit message, > > any possible race outcome is covered by the subsequent checks. > > Okay, so what is the point to call READ_ONCE()? > > > > > > > { > > > > + rcu_read_unlock(); > > > > + kfree_skb(skb); > > > > + return -ECONNREFUSED; > > > > > > This is a generic send_pkt, is it okay to return ECONNREFUSED in any > > > case? > > > > EHOSTUNREACH would probably be better. > > All the current send_pkt functions only return ENODEV, but it has different > > semantics: they mean that the local device isn't yet ready, while there we're > > dealing with the opposite end not being ready. > > In the AF_VSOCK prespective, I see ENODEV like the transport is not ready, > so I think it can eventually fit here too, but also EHOSTUNREACH is fine, > for sure better than ECONNREFUSED. > > Thanks, > Stefano I think it's worth trying to do the same thing with e.g. TCP and see what error, if any, we get. Match that. > > > > Best regards, Polina. > > > > > > > > Thanks, > > > Stefano > > > > > > > + } > > > > + > > > > if (virtio_vsock_skb_reply(skb)) > > > > atomic_inc(&vsock->queued_replies); > > > > > > > > > > > > base-commit: 8ab992f815d6736b5c7a6f5fd7bfe7bc106bb3dc > > > > -- > > > > 2.53.0 > > > >