Re: [PATCH] KVM: arm64: Fix smp_processor_id() call in preemptible context

[Sean Christopherson <seanjc@google.com>]

On Tue, Jun 06, 2023, Oliver Upton wrote:
> The call from a preemptible context is intentional, so this really
> should just be raw_smp_processor_id(). Do you mind if we fix it with the
> following?

...

> Nonetheless, there's no functional requirement for disabling preemption,
> as the cpu # is only used to walk the arm_pmus list. Fix it by using
> raw_smp_processor_id() instead.

As a partial outsider, that needs an explanation, and the code could really use a
comment.  I assume KVM's ABI is that it's userspace's responsibility to ensure that
the CPU(s) used for KVM_RUN is compatible with the CPU used for KVM_ARM_VCPU_PMU_V3_CTRL,
but neither the original changelog nor the above state that, nor does anything
explain what happens if userspace doesn't uphold its side of things.  That stuff
might be covered in documentation somewhere, but for someone just looking at git
blame, this is all very magical.

> Fixes: 1c913a1c35aa ("KVM: arm64: Iterate arm_pmus list to probe for default PMU")
> Reported-by: Sebastian Ott <sebott@redhat.com>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/pmu-emul.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index 491ca7eb2a4c..933a6331168b 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -700,7 +700,7 @@ static struct arm_pmu *kvm_pmu_probe_armpmu(void)
>  
>  	mutex_lock(&arm_pmus_lock);
>  
> -	cpu = smp_processor_id();
> +	cpu = raw_smp_processor_id();
>  	list_for_each_entry(entry, &arm_pmus, entry) {
>  		tmp = entry->arm_pmu;
>  
> 
> base-commit: 9561de3a55bed6bdd44a12820ba81ec416e705a7
> -- 
> 2.41.0.rc0.172.g3f132b7071-goog
> 

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