kvm: i386: irqchip: take BQL only if there is an interrupt

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 <imammedo@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20250814160600.2327672-8-imammedo@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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);
         }
     }
 
@@ -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();
     }
 }