Re: [PATCH 4/7] KVM: selftests: Test consistency of CPUID with num of Fixed counters

[Sean Christopherson]

On Thu, Mar 23, 2023, Like Xu wrote:
> From: Jinrong Liang <cloudliang@xxxxxxxxxxx>
> 
> Add test to check if non-existent counters can be accessed in guest after
> determining the number of Intel generic performance counters by CPUID.
> Per SDM, fixed-function performance counter 'i' is supported if ECX[i] ||
> (EDX[4:0] > i). KVM doesn't emulate more counters than it can support.
> 
> Co-developed-by: Like Xu <likexu@xxxxxxxxxxx>
> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> Signed-off-by: Jinrong Liang <cloudliang@xxxxxxxxxxx>
> ---
>  .../selftests/kvm/x86_64/pmu_cpuid_test.c     | 68 +++++++++++++++++++
>  1 file changed, 68 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/x86_64/pmu_cpuid_test.c b/tools/testing/selftests/kvm/x86_64/pmu_cpuid_test.c
> index 50902187d2c9..c934144be287 100644
> --- a/tools/testing/selftests/kvm/x86_64/pmu_cpuid_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/pmu_cpuid_test.c
> @@ -74,6 +74,22 @@ static uint8_t kvm_gp_ctrs_num(void)
>  	return (kvm_entry->eax & GP_CTR_NUM_MASK) >> GP_CTR_NUM_OFS_BIT;
>  }
>  
> +static uint8_t kvm_fixed_ctrs_num(void)
> +{
> +	const struct kvm_cpuid_entry2 *kvm_entry;
> +
> +	kvm_entry = get_cpuid_entry(kvm_get_supported_cpuid(), 0xa, 0);
> +	return kvm_entry->edx & FIXED_CTR_NUM_MASK;
> +}
> +
> +static uint32_t kvm_fixed_ctrs_bitmask(void)
> +{
> +	const struct kvm_cpuid_entry2 *kvm_entry;
> +
> +	kvm_entry = get_cpuid_entry(kvm_get_supported_cpuid(), 0xa, 0);
> +	return kvm_entry->ecx;
> +}

KVM_X86_CPU_PROPERTY

>  static struct kvm_vcpu *new_vcpu(void *guest_code)
>  {
>  	struct kvm_vm *vm;
> @@ -230,6 +246,39 @@ static void test_oob_gp_counter_setup(struct kvm_vcpu *vcpu, uint8_t eax_gp_num,
>  	vm_install_exception_handler(vcpu->vm, GP_VECTOR, guest_gp_handler);
>  }
>  
> +static uint64_t test_oob_fixed_counter_setup(struct kvm_vcpu *vcpu,
> +					     uint8_t edx_fix_num,
> +					     uint32_t fixed_bitmask)
> +{
> +	struct kvm_cpuid_entry2 *entry;
> +	uint32_t ctr_msr = MSR_CORE_PERF_FIXED_CTR0;
> +	uint8_t idx = edx_fix_num;
> +	bool is_supported = true;

No need to initialize "true", it's explicitly set below.

> +	uint64_t ret = 0xffffULL;
> +
> +	entry = vcpu_get_cpuid_entry(vcpu, 0xa);
> +	entry->ecx = fixed_bitmask;
> +	entry->edx = (entry->edx & ~FIXED_CTR_NUM_MASK) | edx_fix_num;
> +	vcpu_set_cpuid(vcpu);
> +
> +	/* Per Intel SDM, FixCtr[i]_is_supported := ECX[i] || (EDX[4:0] > i). */
> +	is_supported = (entry->ecx & BIT_ULL(idx) ||
> +			((entry->edx & FIXED_CTR_NUM_MASK) > idx));
> +
> +	/* KVM doesn't emulate more fixed counters than it can support. */
> +	if (idx >= kvm_fixed_ctrs_num())
> +		is_supported = false;

Why not this?

	is_supported = idx < kvm_fixed_ctrs_num() &&
		       <CPUID entry stuff>;
> +
> +	if (!is_supported) {
> +		vm_install_exception_handler(vcpu->vm, GP_VECTOR, guest_gp_handler);
> +		ret = GP_VECTOR;
> +	}

Same comments as the previous patch(es).