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Fri, 29 Aug 2025 06:01:21 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGSBuU5yy7Mbc2ZujWUqM7uq+Zevh2Of9cZsRwLKDJSP0n7lyT5LILB49ctQmqQ6g17DxWDlw== X-Received: by 2002:a5d:5d87:0:b0:3c8:f348:7882 with SMTP id ffacd0b85a97d-3c8f34887d8mr15009694f8f.54.1756472479781; Fri, 29 Aug 2025 06:01:19 -0700 (PDT) From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: Igor Mammedov , Peter Xu Subject: [PULL 27/28] kvm: i386: irqchip: take BQL only if there is an interrupt Date: Fri, 29 Aug 2025 14:59:34 +0200 Message-ID: <20250829125935.1526984-28-pbonzini@redhat.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20250829125935.1526984-1-pbonzini@redhat.com> References: <20250829125935.1526984-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1756569694948124100 Content-Type: text/plain; charset="utf-8" From: Igor Mammedov when kernel-irqchip=3Dsplit is used, QEMU still hits BQL contention issue when reading ACPI PM/HPET timers (despite of timer[s] access being lock-less). So Windows with more than 255 cpus is still not able to boot (since it requires iommu -> split irqchip). Problematic path is in kvm_arch_pre_run() where BQL is taken unconditionally when split irqchip is in use. There are a few parts that BQL protects there: 1. interrupt check and injecting however we do not take BQL when checking for pending interrupt (even within the same function), so the patch takes the same approach for cpu->interrupt_request checks and takes BQL only if there is a job to do. 2. request_interrupt_window access CPUState::kvm_run::request_interrupt_window doesn't need BQL as it's accessed by its own vCPU thread. 3. cr8/cpu_get_apic_tpr access the same (as #2) applies to CPUState::kvm_run::cr8, and APIC registers are also cached/synced (get/put) within the vCPU thread it belongs to. Taking BQL only when is necessary, eleminates BQL bottleneck on IO/MMIO only exit path, improoving latency by 80% on HPET micro benchmark. This lets Windows to boot succesfully (in case hv-time isn't used) when more than 255 vCPUs are in use. Signed-off-by: Igor Mammedov Reviewed-by: Peter Xu Link: https://lore.kernel.org/r/20250814160600.2327672-8-imammedo@redhat.com Signed-off-by: Paolo Bonzini --- target/i386/kvm/kvm.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-) diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c index a7b5c8f81bc..306430a0521 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -5478,9 +5478,6 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *= run) } } =20 - if (!kvm_pic_in_kernel()) { - bql_lock(); - } =20 /* Force the VCPU out of its inner loop to process any INIT requests * or (for userspace APIC, but it is cheap to combine the checks here) @@ -5489,10 +5486,10 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run= *run) if (cpu_test_interrupt(cpu, CPU_INTERRUPT_INIT | CPU_INTERRUPT_TPR)) { if (cpu_test_interrupt(cpu, CPU_INTERRUPT_INIT) && !(env->hflags & HF_SMM_MASK)) { - cpu->exit_request =3D 1; + qatomic_set(&cpu->exit_request, 1); } if (cpu_test_interrupt(cpu, CPU_INTERRUPT_TPR)) { - cpu->exit_request =3D 1; + qatomic_set(&cpu->exit_request, 1); } } =20 @@ -5503,6 +5500,8 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *= run) (env->eflags & IF_MASK)) { int irq; =20 + bql_lock(); + cpu->interrupt_request &=3D ~CPU_INTERRUPT_HARD; irq =3D cpu_get_pic_interrupt(env); if (irq >=3D 0) { @@ -5517,6 +5516,7 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *= run) strerror(-ret)); } } + bql_unlock(); } =20 /* If we have an interrupt but the guest is not ready to receive an @@ -5531,8 +5531,6 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *= run) =20 DPRINTF("setting tpr\n"); run->cr8 =3D cpu_get_apic_tpr(x86_cpu->apic_state); - - bql_unlock(); } } =20 --=20 2.51.0