Re: [PATCH v4 7/8] kvm: i386: irqchip: take BQL only if there is an interrupt

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 14/8/25 18:05, 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 <imammedo@redhat.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> ---
> v3:
>    * drop net needed pair of () in cpu->interrupt_request & CPU_INTERRUPT_HARD
>      check
>    * Paolo Bonzini <pbonzini@redhat.com>
>       * 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(-)


>       /* 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);
>           }
>       }

Interesting. IMHO to avoid future similar problems, we should declare
CPUState::exit_request a "private" field and access it via a helper,
where atomicity is enforced.

The only non-accelerator use is in PPC sPAPR:

hw/ppc/spapr_hcall.c:512:        cs->exit_request = 1;
hw/ppc/spapr_hcall.c:534:    cs->exit_request = 1;
hw/ppc/spapr_hcall.c:627:    cs->exit_request = 1;

FYI last week we noticed a similar problem with CPUState::thread_kicked
when using HVF and I'll post in a few days a series containing this
change:

-- >8 --
diff --git a/system/cpus.c b/system/cpus.c
index 26fb3bd69c3..39daf85bae7 100644
--- a/system/cpus.c
+++ b/system/cpus.c
@@ -464,10 +464,10 @@ void qemu_wait_io_event(CPUState *cpu)

  void cpus_kick_thread(CPUState *cpu)
  {
-    if (cpu->thread_kicked) {
+    if (qatomic_read(&cpu->thread_kicked)) {
          return;
      }
-    cpu->thread_kicked = true;
+    qatomic_set(&cpu->thread_kicked, true);

---

It only affects HVF because this is the single access to thread_kicked
out of accelerator code:

$ git grep -w thread_kicked
include/hw/core/cpu.h:484:    bool thread_kicked;
system/cpus.c:449:    qatomic_set_mb(&cpu->thread_kicked, false);
system/cpus.c:476:    if (cpu->thread_kicked) {
system/cpus.c:479:    cpu->thread_kicked = true;
target/arm/hvf/hvf.c:1825:    qatomic_set_mb(&cpu->thread_kicked, false);

(Call introduced in commit 219c101fa7f "arm/hvf: Add a WFI handler").