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Mon, 9 Jan 2023 16:03:43 +0000 (GMT) Date: Mon, 9 Jan 2023 17:03:41 +0100 From: Claudio Imbrenda To: Heiko Carstens Cc: Christian Borntraeger , Janosch Frank , David Hildenbrand , kvm@vger.kernel.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] KVM: s390: interrupt: use READ_ONCE() before cmpxchg() Message-ID: <20230109170341.6d3edc7b@p-imbrenda> In-Reply-To: <20230109145456.2895385-1-hca@linux.ibm.com> References: <20230109145456.2895385-1-hca@linux.ibm.com> Organization: IBM X-Mailer: Claws Mail 4.1.1 (GTK 3.24.35; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 5T46g9dGXCgn6OyAowMtmuMtyNQ7Z96M X-Proofpoint-GUID: SqoaB9pIu61Qn1_q_Bvu7ue_YjBHWEbu X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.923,Hydra:6.0.545,FMLib:17.11.122.1 definitions=2023-01-09_09,2023-01-09_01,2022-06-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1011 lowpriorityscore=0 mlxlogscore=844 phishscore=0 adultscore=0 mlxscore=0 suspectscore=0 impostorscore=0 priorityscore=1501 bulkscore=0 spamscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2212070000 definitions=main-2301090116 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Mon, 9 Jan 2023 15:54:56 +0100 Heiko Carstens wrote: > Use READ_ONCE() before cmpxchg() to prevent that the compiler generates > code that fetches the to be compared old value several times from memory. > > Signed-off-by: Heiko Carstens Reviewed-by: Claudio Imbrenda > --- > > v2: make it compile > > arch/s390/kvm/interrupt.c | 12 ++++++++---- > 1 file changed, 8 insertions(+), 4 deletions(-) > > diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c > index 1dae78deddf2..ab26aa53ee37 100644 > --- a/arch/s390/kvm/interrupt.c > +++ b/arch/s390/kvm/interrupt.c > @@ -83,8 +83,9 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id) > struct esca_block *sca = vcpu->kvm->arch.sca; > union esca_sigp_ctrl *sigp_ctrl = > &(sca->cpu[vcpu->vcpu_id].sigp_ctrl); > - union esca_sigp_ctrl new_val = {0}, old_val = *sigp_ctrl; > + union esca_sigp_ctrl new_val = {0}, old_val; > > + old_val = READ_ONCE(*sigp_ctrl); > new_val.scn = src_id; > new_val.c = 1; > old_val.c = 0; > @@ -95,8 +96,9 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id) > struct bsca_block *sca = vcpu->kvm->arch.sca; > union bsca_sigp_ctrl *sigp_ctrl = > &(sca->cpu[vcpu->vcpu_id].sigp_ctrl); > - union bsca_sigp_ctrl new_val = {0}, old_val = *sigp_ctrl; > + union bsca_sigp_ctrl new_val = {0}, old_val; > > + old_val = READ_ONCE(*sigp_ctrl); > new_val.scn = src_id; > new_val.c = 1; > old_val.c = 0; > @@ -126,16 +128,18 @@ static void sca_clear_ext_call(struct kvm_vcpu *vcpu) > struct esca_block *sca = vcpu->kvm->arch.sca; > union esca_sigp_ctrl *sigp_ctrl = > &(sca->cpu[vcpu->vcpu_id].sigp_ctrl); > - union esca_sigp_ctrl old = *sigp_ctrl; > + union esca_sigp_ctrl old; > > + old = READ_ONCE(*sigp_ctrl); > expect = old.value; > rc = cmpxchg(&sigp_ctrl->value, old.value, 0); > } else { > struct bsca_block *sca = vcpu->kvm->arch.sca; > union bsca_sigp_ctrl *sigp_ctrl = > &(sca->cpu[vcpu->vcpu_id].sigp_ctrl); > - union bsca_sigp_ctrl old = *sigp_ctrl; > + union bsca_sigp_ctrl old; > > + old = READ_ONCE(*sigp_ctrl); > expect = old.value; > rc = cmpxchg(&sigp_ctrl->value, old.value, 0); > }