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envelope-from=peterx@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_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Aug 08, 2025 at 02:01:35PM +0200, Igor Mammedov wrote: > when kernel-irqchip=split 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 > --- > v3: > * drop net needed pair of () in cpu->interrupt_request & CPU_INTERRUPT_HARD > check > * Paolo Bonzini > * don't take BQL when setting exit_request, use qatomic_set() instead > * after above simplification take/release BQL unconditionally > * drop smp_mb() after run->cr8/run->request_interrupt_window update > --- > 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 a7b5c8f81b..306430a052 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) > } > } > > - if (!kvm_pic_in_kernel()) { > - bql_lock(); > - } > > /* 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 = 1; > + qatomic_set(&cpu->exit_request, 1); > } > if (cpu_test_interrupt(cpu, CPU_INTERRUPT_TPR)) { > - cpu->exit_request = 1; > + qatomic_set(&cpu->exit_request, 1); I still see plenty of cases where exit_request will be updated without qatomic_set().. There's even one in kvm.c (kvm_arch_process_async_events) which is the same file. Maybe it would make more sense to move these two changes separately, e.g. a single patch changing tree-wide exit_request setters to use qatomic_set()? But not that it's a huge deal.. Reviewed-by: Peter Xu > } > } > > @@ -5503,6 +5500,8 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *run) > (env->eflags & IF_MASK)) { > int irq; > > + bql_lock(); > + > cpu->interrupt_request &= ~CPU_INTERRUPT_HARD; > irq = cpu_get_pic_interrupt(env); > if (irq >= 0) { > @@ -5517,6 +5516,7 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *run) > strerror(-ret)); > } > } > + bql_unlock(); > } > > /* 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) > > DPRINTF("setting tpr\n"); > run->cr8 = cpu_get_apic_tpr(x86_cpu->apic_state); > - > - bql_unlock(); > } > } > > -- > 2.47.1 > -- Peter Xu