Re: [PATCH 3/4] KVM: x86: Add a VM stat exposing when KVM PIT is set to reinject mode

[Vasant Hegde <vasant.hegde@amd.com>]

On 4/29/2024 9:27 PM, Alejandro Jimenez wrote:
> Add a stat to query when PIT is in reinject mode, which can have a large
> performance impact due to disabling SVM AVIC.
> When using in-kernel irqchip, QEMU and KVM default to creating a PIT in
> reinject mode, since this is necessary for old guest operating systems that
> use the PIT for timing. Unfortunately, reinject mode relies on EOI
> interception and so SVM AVIC must be inhibited when the PIT is set up using
> this mode.
> 
> Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>

Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>

-Vasant

> ---
>  arch/x86/include/asm/kvm_host.h | 1 +
>  arch/x86/kvm/i8254.c            | 2 ++
>  arch/x86/kvm/x86.c              | 3 ++-
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index f3b40cfebec4..e7e3213cefae 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1535,6 +1535,7 @@ struct kvm_vm_stat {
>  	u64 max_mmu_page_hash_collisions;
>  	u64 max_mmu_rmap_size;
>  	u64 synic_auto_eoi_used;
> +	u64 pit_reinject_mode;
>  };
>  
>  struct kvm_vcpu_stat {
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> index cd57a517d04a..44e593e909a1 100644
> --- a/arch/x86/kvm/i8254.c
> +++ b/arch/x86/kvm/i8254.c
> @@ -316,6 +316,8 @@ void kvm_pit_set_reinject(struct kvm_pit *pit, bool reinject)
>  		kvm_unregister_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
>  	}
>  
> +	kvm->stat.pit_reinject_mode = reinject;
> +
>  	atomic_set(&ps->reinject, reinject);
>  }
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 27e339133068..03cb933920cb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -257,7 +257,8 @@ const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
>  	STATS_DESC_ICOUNTER(VM, nx_lpage_splits),
>  	STATS_DESC_PCOUNTER(VM, max_mmu_rmap_size),
>  	STATS_DESC_PCOUNTER(VM, max_mmu_page_hash_collisions),
> -	STATS_DESC_IBOOLEAN(VM, synic_auto_eoi_used)
> +	STATS_DESC_IBOOLEAN(VM, synic_auto_eoi_used),
> +	STATS_DESC_IBOOLEAN(VM, pit_reinject_mode)
>  };
>  
>  const struct kvm_stats_header kvm_vm_stats_header = {