linux-kernel - Re: [PATCH] selftests: kvm/x86: test if it checks all the bits in the LBR

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Message-ID: <YuxVnDif6UMcFZ5I@google.com>
Date:   Thu, 4 Aug 2022 23:26:20 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests: kvm/x86: test if it checks all the bits in
 the LBR_FMT bit-field

On Thu, Aug 04, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> User space only enable guest LBR feature when the exactly supported
> LBR format value is initialized to the MSR_IA32_PERF_CAPABILITIES.
> The input is also invalid if only partially supported bits are set.
> 
> Note for PEBS feature, the PEBS_FORMAT bit field is the primary concern,
> thus if the PEBS_FORMAT input is empty, the other bits check about PEBS
> (like PEBS_TRAP or ARCH_REG) will be ignored.
> 
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c b/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c
> index 6ec901dab61e..98483947f921 100644
> --- a/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c
> @@ -13,6 +13,7 @@
>  
>  #define _GNU_SOURCE /* for program_invocation_short_name */
>  #include <sys/ioctl.h>
> +#include <linux/bitmap.h>
>  
>  #include "kvm_util.h"
>  #include "vmx.h"
> @@ -56,7 +57,7 @@ int main(int argc, char *argv[])
>  	const struct kvm_cpuid_entry2 *entry_a_0;
>  	struct kvm_vm *vm;
>  	struct kvm_vcpu *vcpu;
> -	int ret;
> +	int ret, bit;
>  	union cpuid10_eax eax;
>  	union perf_capabilities host_cap;
>  
> @@ -97,6 +98,12 @@ int main(int argc, char *argv[])
>  	ret = _vcpu_set_msr(vcpu, MSR_IA32_PERF_CAPABILITIES, 0x30);
>  	TEST_ASSERT(ret == 0, "Bad PERF_CAPABILITIES didn't fail.");
>  
> +	/* testcase 4, reject LBR_FMT if only partially supported bits are set */
> +	for_each_set_bit(bit, (unsigned long *)&host_cap.capabilities, 6) {
> +		ret = _vcpu_set_msr(vcpu, MSR_IA32_PERF_CAPABILITIES, BIT_ULL(bit));
> +		TEST_ASSERT(ret == 0, "Bad PERF_CAPABILITIES didn't fail.");
> +	}

Rather than add another testcase and target only a subset of possible values, what
about replacing (fixing IMO) testcase #3 with fully exhaustive approach?

---
From: Sean Christopherson <seanjc@...gle.com>
Date: Thu, 4 Aug 2022 12:18:15 -0700
Subject: [PATCH] KVM: selftests: Test all possible "invalid"
 PERF_CAPABILITIES.LBR_FMT vals

Test all possible input values to verify that KVM rejects all values
except the exact host value.  Due to the LBR format affecting the core
functionality of LBRs, KVM can't emulate "other" formats, so even though
there are a variety of legal values, KVM should reject anything but an
exact host match.

Suggested-by: Like Xu <like.xu.linux@...il.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 .../selftests/kvm/x86_64/vmx_pmu_caps_test.c    | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c b/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c
index 6ec901dab61e..069589c52f41 100644
--- a/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c
+++ b/tools/testing/selftests/kvm/x86_64/vmx_pmu_caps_test.c
@@ -59,6 +59,7 @@ int main(int argc, char *argv[])
 	int ret;
 	union cpuid10_eax eax;
 	union perf_capabilities host_cap;
+	uint64_t val;

 	host_cap.capabilities = kvm_get_feature_msr(MSR_IA32_PERF_CAPABILITIES);
 	host_cap.capabilities &= (PMU_CAP_FW_WRITES | PMU_CAP_LBR_FMT);
@@ -91,11 +92,17 @@ int main(int argc, char *argv[])
 	vcpu_set_msr(vcpu, MSR_IA32_PERF_CAPABILITIES, host_cap.lbr_format);
 	ASSERT_EQ(vcpu_get_msr(vcpu, MSR_IA32_PERF_CAPABILITIES), (u64)host_cap.lbr_format);

-	/* testcase 3, check invalid LBR format is rejected */
-	/* Note, on Arch LBR capable platforms, LBR_FMT in perf capability msr is 0x3f,
-	 * to avoid the failure, use a true invalid format 0x30 for the test. */
-	ret = _vcpu_set_msr(vcpu, MSR_IA32_PERF_CAPABILITIES, 0x30);
-	TEST_ASSERT(ret == 0, "Bad PERF_CAPABILITIES didn't fail.");
+	/*
+	 * Testcase 3, check that an "invalid" LBR format is rejected.  Only an
+	 * exact match of the host's format (and 0/disabled) is allowed.
+	 */
+	for (val = 1; val <= PMU_CAP_LBR_FMT; val++) {
+		if (val == (host_cap.capabilities & PMU_CAP_LBR_FMT))
+			continue;
+
+		ret = _vcpu_set_msr(vcpu, MSR_IA32_PERF_CAPABILITIES, val);
+		TEST_ASSERT(!ret, "Bad LBR FMT = 0x%lx didn't fail", val);
+	}

 	printf("Completed perf capability tests.\n");
 	kvm_vm_free(vm);

base-commit: 93472b79715378a2386598d6632c654a2223267b
--