Re: [PATCH v6 16/25] KVM: x86: Report KVM supported CET MSRs as to-be-saved

[Maxim Levitsky]

On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> Add CET MSRs to the list of MSRs reported to userspace if the feature,
> i.e. IBT or SHSTK, associated with the MSRs is supported by KVM.
> 
> SSP can only be read via RDSSP. Writing even requires destructive and
> potentially faulting operations such as SAVEPREVSSP/RSTORSSP or
> SETSSBSY/CLRSSBSY. Let the host use a pseudo-MSR that is just a wrapper
> for the GUEST_SSP field of the VMCS.

Fake MSR just feels wrong for the future generations of readers of this code. 
This is not a MSR no matter how we look at it, and KVM never supported such
fake MSRs - this is the first one.

I'll say its better to define a new ioctl for this register,
or if you are feeling adventurous, you can try to add support for 
KVM_GET_ONE_REG/KVM_SET_ONE_REG which is what at least arm uses
for this purpose.


Also I think it will be better to split this patch into two - first patch that adds new ioctl,
and the second patch will add the normal CET msrs to the list of msrs to be saved.


> 
> Suggested-by: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>  arch/x86/include/uapi/asm/kvm_para.h |  1 +
>  arch/x86/kvm/vmx/vmx.c               |  2 ++
>  arch/x86/kvm/x86.c                   | 18 ++++++++++++++++++
>  3 files changed, 21 insertions(+)
> 
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 6e64b27b2c1e..9864bbcf2470 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -58,6 +58,7 @@
>  #define MSR_KVM_ASYNC_PF_INT	0x4b564d06
>  #define MSR_KVM_ASYNC_PF_ACK	0x4b564d07
>  #define MSR_KVM_MIGRATION_CONTROL	0x4b564d08
> +#define MSR_KVM_SSP	0x4b564d09

Another reason for not doing this - someone will think that this is a KVM PV msr.

>  
>  struct kvm_steal_time {
>  	__u64 steal;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 72e3943f3693..9409753f45b0 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7009,6 +7009,8 @@ static bool vmx_has_emulated_msr(struct kvm *kvm, u32 index)
>  	case MSR_AMD64_TSC_RATIO:
>  		/* This is AMD only.  */
>  		return false;
> +	case MSR_KVM_SSP:
> +		return kvm_cpu_cap_has(X86_FEATURE_SHSTK);
>  	default:
>  		return true;
>  	}
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index dda9c7141ea1..73b45351c0fc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1476,6 +1476,9 @@ static const u32 msrs_to_save_base[] = {
>  
>  	MSR_IA32_XFD, MSR_IA32_XFD_ERR,
>  	MSR_IA32_XSS,
> +	MSR_IA32_U_CET, MSR_IA32_S_CET,
> +	MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP, MSR_IA32_PL2_SSP,
> +	MSR_IA32_PL3_SSP, MSR_IA32_INT_SSP_TAB,
>  };
>  
>  static const u32 msrs_to_save_pmu[] = {
> @@ -1576,6 +1579,7 @@ static const u32 emulated_msrs_all[] = {
>  
>  	MSR_K7_HWCR,
>  	MSR_KVM_POLL_CONTROL,
> +	MSR_KVM_SSP,
>  };
>  
>  static u32 emulated_msrs[ARRAY_SIZE(emulated_msrs_all)];
> @@ -7241,6 +7245,20 @@ static void kvm_probe_msr_to_save(u32 msr_index)
>  		if (!kvm_caps.supported_xss)
>  			return;
>  		break;
> +	case MSR_IA32_U_CET:
> +	case MSR_IA32_S_CET:
> +		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
> +		    !kvm_cpu_cap_has(X86_FEATURE_IBT))
> +			return;
> +		break;
> +	case MSR_IA32_INT_SSP_TAB:
> +		if (!kvm_cpu_cap_has(X86_FEATURE_LM))
> +			return;
> +		fallthrough;
> +	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> +		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
> +			return;
> +		break;
>  	default:
>  		break;
>  	}

Best regards,
	Maxim Levitsky