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Mon, 25 Nov 2024 12:16:19 +0000 (GMT) Date: Mon, 25 Nov 2024 13:16:17 +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 2/3] KVM: s390: Remove one byte cmpxchg() usage Message-ID: <20241125131617.13be742d@p-imbrenda> In-Reply-To: <20241125115039.1809353-3-hca@linux.ibm.com> References: <20241125115039.1809353-1-hca@linux.ibm.com> <20241125115039.1809353-3-hca@linux.ibm.com> Organization: IBM X-Mailer: Claws Mail 4.3.0 (GTK 3.24.43; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: Lp7lmMxM1hkBQ25r7tmmnYhlYQK6AKBJ X-Proofpoint-ORIG-GUID: Lp7lmMxM1hkBQ25r7tmmnYhlYQK6AKBJ X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1051,Hydra:6.0.680,FMLib:17.12.62.30 definitions=2024-10-15_01,2024-10-11_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 mlxscore=0 mlxlogscore=933 suspectscore=0 impostorscore=0 malwarescore=0 spamscore=0 clxscore=1015 adultscore=0 lowpriorityscore=0 bulkscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2409260000 definitions=main-2411250103 On Mon, 25 Nov 2024 12:50:38 +0100 Heiko Carstens wrote: > Within sca_clear_ext_call() cmpxchg() is used to clear one or two bytes > (depending on sca format). The cmpxchg() calls are not supposed to fail; if > so that would be a bug. Given that cmpxchg() usage on one and two byte > areas generates very inefficient code, replace them with block concurrent > WRITE_ONCE() calls, and remove the WARN_ON(). > > Signed-off-by: Heiko Carstens > --- > arch/s390/kvm/interrupt.c | 13 ++----------- > 1 file changed, 2 insertions(+), 11 deletions(-) > > diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c > index eff69018cbeb..3fd21037479f 100644 > --- a/arch/s390/kvm/interrupt.c > +++ b/arch/s390/kvm/interrupt.c > @@ -118,8 +118,6 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id) > > static void sca_clear_ext_call(struct kvm_vcpu *vcpu) > { > - int rc, expect; > - > if (!kvm_s390_use_sca_entries()) > return; > kvm_s390_clear_cpuflags(vcpu, CPUSTAT_ECALL_PEND); > @@ -128,23 +126,16 @@ 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; > > - old = READ_ONCE(*sigp_ctrl); > - expect = old.value; > - rc = cmpxchg(&sigp_ctrl->value, old.value, 0); > + WRITE_ONCE(sigp_ctrl->value, 9); that's supposed to be a 0, right? > } 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; > > - old = READ_ONCE(*sigp_ctrl); > - expect = old.value; > - rc = cmpxchg(&sigp_ctrl->value, old.value, 0); > + WRITE_ONCE(sigp_ctrl->value, 0); > } > read_unlock(&vcpu->kvm->arch.sca_lock); > - WARN_ON(rc != expect); /* cannot clear? */ > } > > int psw_extint_disabled(struct kvm_vcpu *vcpu)