Re: [PATCH] KVM: x86: Add a vCPU stat for #AC exceptions

[Xiaoyao Li]

On 4/26/2023 4:26 PM, Anselm Busse wrote:
> This patch adds a KVM vCPU stat that reflects the number of #AC
> exceptions caused by a guest. This improves the identification and
> debugging of issues that are possibly caused by guests triggering
> split-locks and allows more insides compared to the current situation
> of having only a warning printed when an #AC exception is raised.

Note, on Intel platform, #AC exception has three sources according to 
the latest spec:

1. violation on alignment check when CPL = 3, while CR0.AM and EFLAG.AC 
are set;

2. split lock, when MSR_MEMORY_CTRL.[29] is set;

3. UC lock, when CPUID.0x7_0x2:EDX[16] is 1 and 
MSR_MEMORY_CTRL(0x33).[28] is 1. (see ISE version 048);

you cannot treat every #AC as split lock #AC.

> Signed-off-by: Anselm Busse <abusse@xxxxxxxxxx>
> ---
>   arch/x86/include/asm/kvm_host.h | 1 +
>   arch/x86/kvm/vmx/vmx.c          | 2 ++
>   arch/x86/kvm/x86.c              | 1 +
>   3 files changed, 4 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 808c292ad3f4..b4ab719fbc69 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1523,6 +1523,7 @@ struct kvm_vcpu_stat {
>   	u64 preemption_other;
>   	u64 guest_mode;
>   	u64 notify_window_exits;
> +	u64 split_lock_exceptions;
>   };
>   
>   struct x86_instruction_info;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d2d6e1b6c788..8f48fd8ddead 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5309,6 +5309,8 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>   		kvm_run->debug.arch.exception = ex_no;
>   		break;
>   	case AC_VECTOR:
> +		vmx->vcpu.stat.split_lock_exceptions++;
> +
>   		if (vmx_guest_inject_ac(vcpu)) {
>   			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
>   			return 1;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3d852ce84920..416a1ed6c423 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -297,6 +297,7 @@ const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
>   	STATS_DESC_COUNTER(VCPU, preemption_other),
>   	STATS_DESC_IBOOLEAN(VCPU, guest_mode),
>   	STATS_DESC_COUNTER(VCPU, notify_window_exits),
> +	STATS_DESC_COUNTER(VCPU, split_lock_exceptions),
>   };
>   
>   const struct kvm_stats_header kvm_vcpu_stats_header = {