Re: [PATCH v6 12/25] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Binbin Wu]

On 9/14/2023 2:33 PM, 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 !guest_cpuid_has(),
> write initiated from host is allowed iff the write is reset operaiton,
> i.e., data == 0, reject host_initiated non-reset write and any guest write.
Hi Sean & Polo,
During code review of Enable CET Virtualization v5 patchset, there were
discussions about "do a wholesale cleanup of all the cases that essentially
allow userspace to do KVM_SET_MSR before KVM_SET_CPUID2", i.e. force the 
order
between  KVM_SET_CPUID2 and KVM_SET_MSR, but allow the host_initiated 
path with
default (generally 0) value.
https://lore.kernel.org/kvm/ZM1C+ILRMCfzJxx7@xxxxxxxxxx/
https://lore.kernel.org/kvm/CABgObfbvr8F8g5hJN6jn95m7u7m2+8ACkqO25KAZwRmJ9AncZg@xxxxxxxxxxxxxx/

I can take the task to do the code cleanup.
Before going any further, I want to confirm it is still the direction 
intended,
right?


> 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 |  1 +
>   arch/x86/kvm/cpuid.c            | 15 ++++++++++++++-
>   arch/x86/kvm/x86.c              | 13 +++++++++----
>   3 files changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0fc5e6312e93..d77b030e996c 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -803,6 +803,7 @@ struct kvm_vcpu_arch {
>   
>   	u64 xcr0;
>   	u64 guest_supported_xcr0;
> +	u64 guest_supported_xss;
>   
>   	struct kvm_pio_request pio;
>   	void *pio_data;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 1f206caec559..4e7a820cba62 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);
>   
>   	/*
>   	 * FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1258d1d6dd52..9a616d84bd39 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3795,20 +3795,25 @@ 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: {
> +		bool host_msr_reset = msr_info->host_initiated && data == 0;
> +
> +		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES) &&
> +		    (!host_msr_reset || !msr_info->host_initiated))
>   			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;