linux-kernel - Re: [PATCH v2 06/49] KVM: selftests: Refresh vCPU CPUID cache in __vcpu_get_cpuid

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Message-ID: <2c776a37ac2c2191e37993d2fc435755c95e78cb.camel@redhat.com>
Date: Thu, 04 Jul 2024 20:59:17 -0400
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>,  Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Hou Wenlong
 <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>, Oliver Upton
 <oliver.upton@...ux.dev>, Binbin Wu <binbin.wu@...ux.intel.com>, Yang
 Weijiang <weijiang.yang@...el.com>, Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 06/49] KVM: selftests: Refresh vCPU CPUID cache in
 __vcpu_get_cpuid_entry()

On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> Refresh selftests' CPUID cache in the vCPU structure when querying a CPUID
> entry so that tests don't consume stale data when KVM modifies CPUID as a
> side effect to a completely unrelated change.  E.g. KVM adjusts OSXSAVE in
> response to CR4.OSXSAVE changes.
> 
> Unnecessarily invoking KVM_GET_CPUID is suboptimal, but vcpu->cpuid exists
> to simplify selftests development, not for performance reasons.  And,
> unfortunately, trying to handle the side effects in tests or other flows
> is unpleasant, e.g. selftests could manually refresh if KVM_SET_SREGS is
> successful, but that would still leave a gap with respect to guest CR4
> changes.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  .../testing/selftests/kvm/include/x86_64/processor.h  | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
> index 8eb57de0b587..99aa3dfca16c 100644
> --- a/tools/testing/selftests/kvm/include/x86_64/processor.h
> +++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
> @@ -992,10 +992,17 @@ static inline struct kvm_cpuid2 *allocate_kvm_cpuid2(int nr_entries)
>  void vcpu_init_cpuid(struct kvm_vcpu *vcpu, const struct kvm_cpuid2 *cpuid);
>  void vcpu_set_hv_cpuid(struct kvm_vcpu *vcpu);
>  
> +static inline void vcpu_get_cpuid(struct kvm_vcpu *vcpu)
> +{
> +	vcpu_ioctl(vcpu, KVM_GET_CPUID2, vcpu->cpuid);
> +}
> +
>  static inline struct kvm_cpuid_entry2 *__vcpu_get_cpuid_entry(struct kvm_vcpu *vcpu,
>  							      uint32_t function,
>  							      uint32_t index)
>  {
> +	vcpu_get_cpuid(vcpu);
> +
>  	return (struct kvm_cpuid_entry2 *)get_cpuid_entry(vcpu->cpuid,
>  							  function, index);
>  }
> @@ -1016,7 +1023,7 @@ static inline int __vcpu_set_cpuid(struct kvm_vcpu *vcpu)
>  		return r;
>  
>  	/* On success, refresh the cache to pick up adjustments made by KVM. */
> -	vcpu_ioctl(vcpu, KVM_GET_CPUID2, vcpu->cpuid);
> +	vcpu_get_cpuid(vcpu);
>  	return 0;
>  }
>  
> @@ -1026,7 +1033,7 @@ static inline void vcpu_set_cpuid(struct kvm_vcpu *vcpu)
>  	vcpu_ioctl(vcpu, KVM_SET_CPUID2, vcpu->cpuid);
>  
>  	/* Refresh the cache to pick up adjustments made by KVM. */
> -	vcpu_ioctl(vcpu, KVM_GET_CPUID2, vcpu->cpuid);
> +	vcpu_get_cpuid(vcpu);
>  }
>  
>  void vcpu_set_cpuid_property(struct kvm_vcpu *vcpu,

Hi,

Indeed - fully agree with this, and again very sad that Intel
made their CPUID dynamic - what a hack :(

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky