linux-kernel - Re: [PATCH 02/11] KVM: selftests: Precisely mask off dynamic fields in CPUID test

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Message-ID: <87setci6l6.fsf@redhat.com>
Date: Fri, 04 Oct 2024 11:02:29 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Sean Christopherson
 <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/11] KVM: selftests: Precisely mask off dynamic fields
 in CPUID test

Sean Christopherson <seanjc@...gle.com> writes:

> When comparing vCPU CPUID entries against KVM's supported CPUID, mask off
> only the dynamic fields/bits instead of skipping the entire entry.
> Precisely masking bits isn't meaningfully more difficult than skipping
> entire entries, and will be necessary to maintain test coverage when a
> future commit enables OSXSAVE by default, i.e. makes one bit in all of
> CPUID.0x1 dynamic.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  .../testing/selftests/kvm/x86_64/cpuid_test.c | 61 +++++++++++--------
>  1 file changed, 36 insertions(+), 25 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/x86_64/cpuid_test.c b/tools/testing/selftests/kvm/x86_64/cpuid_test.c
> index fec03b11b059..f7fdcef5fa59 100644
> --- a/tools/testing/selftests/kvm/x86_64/cpuid_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/cpuid_test.c
> @@ -12,17 +12,16 @@
>  #include "kvm_util.h"
>  #include "processor.h"
>  
> -/* CPUIDs known to differ */
> -struct {
> -	u32 function;
> -	u32 index;
> -} mangled_cpuids[] = {
> -	/*
> -	 * These entries depend on the vCPU's XCR0 register and IA32_XSS MSR,
> -	 * which are not controlled for by this test.
> -	 */
> -	{.function = 0xd, .index = 0},
> -	{.function = 0xd, .index = 1},
> +struct cpuid_mask {
> +	union {
> +		struct {
> +			u32 eax;
> +			u32 ebx;
> +			u32 ecx;
> +			u32 edx;
> +		};
> +		u32 regs[4];
> +	};
>  };
>  
>  static void test_guest_cpuids(struct kvm_cpuid2 *guest_cpuid)
> @@ -56,17 +55,23 @@ static void guest_main(struct kvm_cpuid2 *guest_cpuid)
>  	GUEST_DONE();
>  }
>  
> -static bool is_cpuid_mangled(const struct kvm_cpuid_entry2 *entrie)
> +static struct cpuid_mask get_const_cpuid_mask(const struct kvm_cpuid_entry2 *entry)
>  {
> -	int i;
> +	struct cpuid_mask mask;
>  
> -	for (i = 0; i < ARRAY_SIZE(mangled_cpuids); i++) {
> -		if (mangled_cpuids[i].function == entrie->function &&
> -		    mangled_cpuids[i].index == entrie->index)
> -			return true;
> +	memset(&mask, 0xff, sizeof(mask));
> +
> +	switch (entry->function) {
> +	case 0xd:
> +		/*
> +		 * CPUID.0xD.{0,1}.EBX enumerate XSAVE size based on the current
> +		 * XCR0 and IA32_XSS MSR values.
> +		 */
> +		if (entry->index < 2)
> +			mask.ebx = 0;
> +		break;
>  	}
> -
> -	return false;
> +	return mask;
>  }
>  
>  static void compare_cpuids(const struct kvm_cpuid2 *cpuid1,
> @@ -79,6 +84,8 @@ static void compare_cpuids(const struct kvm_cpuid2 *cpuid1,
>  		    "CPUID nent mismatch: %d vs. %d", cpuid1->nent, cpuid2->nent);
>  
>  	for (i = 0; i < cpuid1->nent; i++) {
> +		struct cpuid_mask mask;
> +
>  		e1 = &cpuid1->entries[i];
>  		e2 = &cpuid2->entries[i];
>  
> @@ -88,15 +95,19 @@ static void compare_cpuids(const struct kvm_cpuid2 *cpuid1,
>  			    i, e1->function, e1->index, e1->flags,
>  			    e2->function, e2->index, e2->flags);
>  
> -		if (is_cpuid_mangled(e1))
> -			continue;
> +		/* Mask off dynamic bits, e.g. OSXSAVE, when comparing entries. */
> +		mask = get_const_cpuid_mask(e1);
>  
> -		TEST_ASSERT(e1->eax == e2->eax && e1->ebx == e2->ebx &&
> -			    e1->ecx == e2->ecx && e1->edx == e2->edx,
> +		TEST_ASSERT((e1->eax & mask.eax) == (e2->eax & mask.eax) &&
> +			    (e1->ebx & mask.ebx) == (e2->ebx & mask.ebx) &&
> +			    (e1->ecx & mask.ecx) == (e2->ecx & mask.ecx) &&
> +			    (e1->edx & mask.edx) == (e2->edx & mask.edx),
>  			    "CPUID 0x%x.%x differ: 0x%x:0x%x:0x%x:0x%x vs 0x%x:0x%x:0x%x:0x%x",
>  			    e1->function, e1->index,
> -			    e1->eax, e1->ebx, e1->ecx, e1->edx,
> -			    e2->eax, e2->ebx, e2->ecx, e2->edx);
> +			    e1->eax & mask.eax, e1->ebx & mask.ebx,
> +			    e1->ecx & mask.ecx, e1->edx & mask.edx,
> +			    e2->eax & mask.eax, e2->ebx & mask.ebx,
> +			    e2->ecx & mask.ecx, e2->edx & mask.edx);
>  	}
>  }

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly