linux-kernel - Re: [PATCH 1/7] KVM: x86: add SPEC_CTRL and IBPB

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Message-ID: <dddec943-302a-19a7-e45f-da2aaefb0b8e@redhat.com>
Date:   Tue, 9 Jan 2018 12:14:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     jmattson@...gle.com, aliguori@...zon.com, thomas.lendacky@....com,
        dwmw@...zon.co.uk, bp@...en8.de
Subject: Re: [PATCH 1/7] KVM: x86: add SPEC_CTRL and IBPB_SUPPORT accessors

On 08.01.2018 19:08, Paolo Bonzini wrote:
> As an interim measure until SPEC_CTRL is supported by upstream
> Linux in cpufeatures, add a function that lets vmx.c and svm.c
> know whether to save/restore MSR_IA32_SPEC_CTRL.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/cpuid.c |  3 ---
>  arch/x86/kvm/cpuid.h | 22 ++++++++++++++++++++++
>  2 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 8e9a07c557f1..767af697c20c 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -67,9 +67,6 @@ u64 kvm_supported_xcr0(void)
>  
>  #define F(x) bit(X86_FEATURE_##x)
>  
> -/* These are scattered features in cpufeatures.h. */
> -#define KVM_CPUID_BIT_AVX512_4VNNIW     2
> -#define KVM_CPUID_BIT_AVX512_4FMAPS     3
>  #define KF(x) bit(KVM_CPUID_BIT_##x)
>  
>  int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index c2cea6651279..8d04ccf177ce 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -155,6 +155,28 @@ static inline int guest_cpuid_stepping(struct kvm_vcpu *vcpu)
>  	return x86_stepping(best->eax);
>  }
>  
> +/* These are scattered features in cpufeatures.h. */
> +#define KVM_CPUID_BIT_AVX512_4VNNIW	2
> +#define KVM_CPUID_BIT_AVX512_4FMAPS	3
> +#define KVM_CPUID_BIT_SPEC_CTRL		26
> +#define KVM_CPUID_BIT_STIBP		27

I can see that STIBP is never checked in KVM code but only forwarded to
the guest if available.

I am wondering if we would have to check against that, too, before
issuing a FEATURE_SET_IBPB.

(can somebody point me at the intel documentation?)

> +
> +/* CPUID[eax=0x80000008].ebx */
> +#define KVM_CPUID_BIT_IBPB_SUPPORT	12
> +
> +static inline bool cpu_has_spec_ctrl(void)
> +{
> +	u32 eax, ebx, ecx, edx;
> +	cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
> +
> +	return edx & bit(KVM_CPUID_BIT_SPEC_CTRL);
> +}
> +
> +static inline bool cpu_has_ibpb_support(void)
> +{
> +	return cpuid_ebx(0x80000008) & bit(KVM_CPUID_BIT_IBPB_SUPPORT);
> +}
> +
>  static inline bool supports_cpuid_fault(struct kvm_vcpu *vcpu)
>  {
>  	return vcpu->arch.msr_platform_info & MSR_PLATFORM_INFO_CPUID_FAULT;
> 


-- 

Thanks,

David / dhildenb