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bh=6L263UDGrn4scR5Lz5TYJ5DyNcjVsmVXQP5VkYdUQZY=; b=II6jP8Kjm8GeGIIZEMikQabQfRf4acCLmxDUDO+PqHUcEQpEEWCdj8RI4AGSmiSbNnuqUN bEzaq58hR6xOIhY+sc/X3TcgN8fTpmsLzkPCZ9maJToeKo0wHvsEe+yDPenT4SSIrqmNTF t6Pte5m2Zo72GQKGIS7BFu6NGPX6jUg= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-537-f6NNffpLPg20KtAc2hLRxA-1; Tue, 16 Feb 2021 09:21:48 -0500 X-MC-Unique: f6NNffpLPg20KtAc2hLRxA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 73C341936B73; Tue, 16 Feb 2021 14:21:47 +0000 (UTC) Received: from thuth.remote.csb (ovpn-112-157.ams2.redhat.com [10.36.112.157]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3A54C60C9C; Tue, 16 Feb 2021 14:21:46 +0000 (UTC) Subject: Re: [PATCH] pc-bios/s390-ccw: Use memory barriers in virtio code To: Cornelia Huck References: <20210216110056.1228582-1-thuth@redhat.com> <20210216124757.4eb664e9.cohuck@redhat.com> From: Thomas Huth Message-ID: Date: Tue, 16 Feb 2021 15:21:45 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <20210216124757.4eb664e9.cohuck@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=thuth@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Christian Borntraeger , qemu-s390x@nongnu.org, qemu-devel@nongnu.org, Peter Maydell Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 16/02/2021 12.47, Cornelia Huck wrote: > On Tue, 16 Feb 2021 12:00:56 +0100 > Thomas Huth wrote: > >> According to the virtio specification, a memory barrier should be >> used before incrementing the idx field in the "available" ring. >> So far, we did not do this in the s390-ccw bios yet, but recently >> Peter Maydell saw problems with the s390-ccw bios when running >> the qtests on an aarch64 host (the bios panic'ed with the message: >> "SCSI cannot report LUNs: response VS RESP=09"), which could >> maybe be related to the missing memory barriers. Thus let's add >> those barriers now. Since we've only seen the problem on TCG so far, >> a "bcr 14,0" should be sufficient here to trigger the tcg_gen_mb() >> in the TCG translate code. >> >> (Note: The virtio spec also talks about using a memory barrier >> *after* incrementing the idx field, but if I understood correctly >> this is only required when using notification suppression - which >> we don't use in the s390-ccw bios here) >> >> Signed-off-by: Thomas Huth >> --- >> pc-bios/s390-ccw/virtio-net.c | 1 + >> pc-bios/s390-ccw/virtio.c | 1 + >> pc-bios/s390-ccw/virtio.h | 2 ++ >> 3 files changed, 4 insertions(+) >> >> diff --git a/pc-bios/s390-ccw/virtio-net.c b/pc-bios/s390-ccw/virtio-net.c >> index 2fcb0a58c5..25598a7a97 100644 >> --- a/pc-bios/s390-ccw/virtio-net.c >> +++ b/pc-bios/s390-ccw/virtio-net.c >> @@ -127,6 +127,7 @@ int recv(int fd, void *buf, int maxlen, int flags) >> >> /* Mark buffer as available to the host again */ >> rxvq->avail->ring[rxvq->avail->idx % rxvq->num] = id; >> + virtio_mb(); >> rxvq->avail->idx = rxvq->avail->idx + 1; >> vring_notify(rxvq); >> >> diff --git a/pc-bios/s390-ccw/virtio.c b/pc-bios/s390-ccw/virtio.c >> index ab49840db8..fb9687f9b3 100644 >> --- a/pc-bios/s390-ccw/virtio.c >> +++ b/pc-bios/s390-ccw/virtio.c >> @@ -154,6 +154,7 @@ void vring_send_buf(VRing *vr, void *p, int len, int flags) >> >> /* Chains only have a single ID */ >> if (!(flags & VRING_DESC_F_NEXT)) { >> + virtio_mb(); > > I think you need to also need barriers for changes to the buffers, as > the spec talks about "manipulating the descriptor table". Which paragraph in the virtio spec are you refering to here? I can't find that part right now... >> vr->avail->idx++; >> } >> } >> diff --git a/pc-bios/s390-ccw/virtio.h b/pc-bios/s390-ccw/virtio.h >> index 19fceb6495..6ac65482a9 100644 >> --- a/pc-bios/s390-ccw/virtio.h >> +++ b/pc-bios/s390-ccw/virtio.h >> @@ -271,6 +271,8 @@ struct VirtioCmd { >> }; >> typedef struct VirtioCmd VirtioCmd; >> >> +#define virtio_mb() asm volatile("bcr 14,0" : : : "memory") > > The bios is built for z900, so you probably need a bcr15 here? I thought about that, too, but for TCG, it currently should not matter since both, 14 and 15, end up with the same code in op_bc() in target/s390x/translate.c. And on a real host, we've never seen this problem to occur, so it should not matter there, too. But if you prefer (e.g. in case somebody tweaks the TCG implementation one day), I can also switch to bcr15 instead. Thomas