Re: [PATCH v7 13/26] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Maxim Levitsky]

On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
> Update CPUID.(EAX=0DH,ECX=1).EBX to reflect current required xstate size
> due to XSS MSR modification.
> CPUID(EAX=0DH,ECX=1).EBX reports the required storage size of all enabled
> xstate features in (XCR0 | IA32_XSS). The CPUID value can be used by guest
> before allocate sufficient xsave buffer.
> 
> Note, KVM does not yet support any XSS based features, i.e. supported_xss
> is guaranteed to be zero at this time.
> 
> Opportunistically modify XSS write access logic as:
> If XSAVES is not enabled in the guest CPUID, forbid setting IA32_XSS msr
> to anything but 0, even if the write is host initiated.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  3 ++-
>  arch/x86/kvm/cpuid.c            | 15 ++++++++++++++-
>  arch/x86/kvm/x86.c              | 16 ++++++++++++----
>  3 files changed, 28 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 499bd42e3a32..f536102f1eca 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -756,7 +756,6 @@ struct kvm_vcpu_arch {
>  	bool at_instruction_boundary;
>  	bool tpr_access_reporting;
>  	bool xfd_no_write_intercept;
> -	u64 ia32_xss;
>  	u64 microcode_version;
>  	u64 arch_capabilities;
>  	u64 perf_capabilities;
> @@ -812,6 +811,8 @@ struct kvm_vcpu_arch {
>  
>  	u64 xcr0;
>  	u64 guest_supported_xcr0;
> +	u64 guest_supported_xss;
> +	u64 ia32_xss;
>  
>  	struct kvm_pio_request pio;
>  	void *pio_data;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0351e311168a..1d9843b34196 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -275,7 +275,8 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
>  	best = cpuid_entry2_find(entries, nent, 0xD, 1);
>  	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>  		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> -		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> +		best->ebx = xstate_required_size(vcpu->arch.xcr0 |
> +						 vcpu->arch.ia32_xss, true);
>  
>  	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
>  	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> @@ -312,6 +313,17 @@ static u64 vcpu_get_supported_xcr0(struct kvm_vcpu *vcpu)
>  	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
>  }
>  
> +static u64 vcpu_get_supported_xss(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +
> +	best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
> +	if (!best)
> +		return 0;
> +
> +	return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
> +}
> +
>  static bool kvm_cpuid_has_hyperv(struct kvm_cpuid_entry2 *entries, int nent)
>  {
>  	struct kvm_cpuid_entry2 *entry;
> @@ -358,6 +370,7 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	}
>  
>  	vcpu->arch.guest_supported_xcr0 = vcpu_get_supported_xcr0(vcpu);
> +	vcpu->arch.guest_supported_xss = vcpu_get_supported_xss(vcpu);
>  
>  	kvm_update_pv_runtime(vcpu);
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f7d4cc61bc55..649a100ffd25 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3901,20 +3901,28 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			vcpu->arch.ia32_tsc_adjust_msr += adj;
>  		}
>  		break;
> -	case MSR_IA32_XSS:
> -		if (!msr_info->host_initiated &&
> -		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> +	case MSR_IA32_XSS: {
> +		/*
> +		 * If KVM reported support of XSS MSR, even guest CPUID doesn't
> +		 * support XSAVES, still allow userspace to set default value(0)
> +		 * to this MSR.
> +		 */
> +		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES) &&
> +		    !(msr_info->host_initiated && data == 0))
>  			return 1;
>  		/*
>  		 * KVM supports exposing PT to the guest, but does not support
>  		 * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
>  		 * XSAVES/XRSTORS to save/restore PT MSRs.
>  		 */
> -		if (data & ~kvm_caps.supported_xss)
> +		if (data & ~vcpu->arch.guest_supported_xss)
>  			return 1;
> +		if (vcpu->arch.ia32_xss == data)
> +			break;
>  		vcpu->arch.ia32_xss = data;
>  		kvm_update_cpuid_runtime(vcpu);
>  		break;
> +	}
>  	case MSR_SMI_COUNT:
>  		if (!msr_info->host_initiated)
>  			return 1;

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky