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Tsirkin" , qemu-s390x@nongnu.org, qemu-devel@nongnu.org Subject: [PATCH 2/2] s390x/pci: fix set_ind_atomic Date: Tue, 16 Jun 2020 06:50:35 +0200 Message-Id: <20200616045035.51641-3-pasic@linux.ibm.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20200616045035.51641-1-pasic@linux.ibm.com> References: <20200616045035.51641-1-pasic@linux.ibm.com> X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216, 18.0.687 definitions=2020-06-15_11:2020-06-15, 2020-06-15 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 bulkscore=0 mlxlogscore=999 adultscore=0 lowpriorityscore=0 mlxscore=0 malwarescore=0 spamscore=0 impostorscore=0 suspectscore=0 clxscore=1015 priorityscore=1501 cotscore=-2147483648 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006160028 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 = 2020/06/16 00:50:54 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: Halil Pasic , Thomas Huth , David Hildenbrand , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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 new old is used to compute the new desired value if the xchg part is not performed. Signed-off-by: Halil Pasic Fixes: 8cba80c3a0 ("s390: Add PCI bus support") Reported-by: Christian Borntraeger --- hw/s390x/s390-pci-bus.c | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-) diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c index c4a4259f0c..80eb957a91 100644 --- a/hw/s390x/s390-pci-bus.c +++ b/hw/s390x/s390-pci-bus.c @@ -637,22 +637,24 @@ static AddressSpace *s390_pci_dma_iommu(PCIBus *bus, void *opaque, int devfn) static uint8_t set_ind_atomic(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) { s390_pci_generate_error_event(ERR_EVENT_AIRERR, 0, 0, 0, 0); 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); - cpu_physical_memory_unmap(ind_addr, len, 1, len); + expected = actual; + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set); + } while (actual != expected); + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len); - return ind_old; + return actual; } static void s390_msi_ctrl_write(void *opaque, hwaddr addr, uint64_t data,