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Mon, 6 Jul 2020 11:23:13 GMT Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0D7E111C058; Mon, 6 Jul 2020 11:23:13 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9C0C111C052; Mon, 6 Jul 2020 11:23:12 +0000 (GMT) Received: from oc2783563651 (unknown [9.145.6.129]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Mon, 6 Jul 2020 11:23:12 +0000 (GMT) Date: Mon, 6 Jul 2020 13:23:11 +0200 From: Halil Pasic To: Cornelia Huck Subject: Re: [PULL 13/14] virtio-ccw: fix virtio_set_ind_atomic Message-ID: <20200706132311.2add044f.pasic@linux.ibm.com> In-Reply-To: <20200703100650.621212-14-cohuck@redhat.com> References: <20200703100650.621212-1-cohuck@redhat.com> <20200703100650.621212-14-cohuck@redhat.com> Organization: IBM X-Mailer: Claws Mail 3.11.1 (GTK+ 2.24.31; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235, 18.0.687 definitions=2020-07-06_08:2020-07-06, 2020-07-06 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 adultscore=0 lowpriorityscore=0 mlxlogscore=999 impostorscore=0 bulkscore=0 priorityscore=1501 cotscore=-2147483648 malwarescore=0 suspectscore=0 mlxscore=0 spamscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2007060086 Received-SPF: pass client-ip=148.163.156.1; envelope-from=pasic@linux.ibm.com; helo=mx0a-001b2d01.pphosted.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/06 07:21:42 X-ACL-Warn: Detected OS = Linux 3.x [generic] [fuzzy] X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN 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: Peter Maydell , qemu-s390x@nongnu.org, Andre Wild , qemu-devel@nongnu.org, Christian Borntraeger Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, 3 Jul 2020 12:06:49 +0200 Cornelia Huck wrote: > From: Halil Pasic > > The atomic_cmpxchg() loop is broken because we occasionally end up with > old and _old having different values (a legit compiler can generate code > that accessed *ind_addr again to pick up a value for _old instead of > using the value of old that was already fetched according to the > rules of the abstract machine). This means the underlying CS instruction > may use a different old (_old) than the one we intended to use if > atomic_cmpxchg() performed the xchg part. > > Let us use volatile to force the rules of the abstract machine for > accesses to *ind_addr. Let us also rewrite the loop so, we that the Michael T. Has pointed out that this sentence is ungrammatical. s/we// would IMHO solve the problem. Can we fix this before it gets merged? > new old is used to compute the new desired value if the xchg part > is not performed. > > Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.") > Reported-by: Andre Wild > Signed-off-by: Halil Pasic > Reviewed-by: Christian Borntraeger > Message-Id: <20200616045035.51641-2-pasic@linux.ibm.com> > Signed-off-by: Cornelia Huck > --- > hw/s390x/virtio-ccw.c | 18 ++++++++++-------- > 1 file changed, 10 insertions(+), 8 deletions(-) > > diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c > index c1f4bb1d331d..3c988a000b5b 100644 > --- a/hw/s390x/virtio-ccw.c > +++ b/hw/s390x/virtio-ccw.c > @@ -786,9 +786,10 @@ static inline VirtioCcwDevice *to_virtio_ccw_dev_fast(DeviceState *d) > static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc, > uint8_t to_be_set) > { > - uint8_t ind_old, ind_new; > + uint8_t expected, actual; > hwaddr len = 1; > - uint8_t *ind_addr; > + /* avoid multiple fetches */ > + uint8_t volatile *ind_addr; > > ind_addr = cpu_physical_memory_map(ind_loc, &len, true); > if (!ind_addr) { > @@ -796,14 +797,15 @@ static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc, > __func__, sch->cssid, sch->ssid, sch->schid); > return -1; > } > + actual = *ind_addr; > do { > - ind_old = *ind_addr; > - ind_new = ind_old | to_be_set; > - } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old); > - trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new); > - cpu_physical_memory_unmap(ind_addr, len, 1, len); > + expected = actual; > + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set); > + } while (actual != expected); > + trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set); > + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len); > > - return ind_old; > + return actual; > } > > static void virtio_ccw_notify(DeviceState *d, uint16_t vector)