Re: [PATCH] KVM: VMX: add MSR_IA32_TSX_CTRL into msrs_to_save

[Sean Christopherson <seanjc@google.com>]

On Wed, May 03, 2023, Mingwei Zhang wrote:
> Add MSR_IA32_TSX_CTRL into msrs_to_save[] to explicitly tell userspace to
> save/restore the register value during migration. Missing this may cause
> userspace that relies on KVM ioctl(KVM_GET_MSR_INDEX_LIST) fail to port the
> value to the target VM.
> 
> Fixes: b07a5c53d42a ("KVM: vmx: use MSR_IA32_TSX_CTRL to hard-disable TSX on guest that lack it")
> Reported-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 237c483b1230..2236cfee4b7a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1431,7 +1431,7 @@ static const u32 msrs_to_save_base[] = {
>  #endif
>  	MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
>  	MSR_IA32_FEAT_CTL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
> -	MSR_IA32_SPEC_CTRL,
> +	MSR_IA32_SPEC_CTRL, MSR_IA32_TSX_CTRL,
>  	MSR_IA32_RTIT_CTL, MSR_IA32_RTIT_STATUS, MSR_IA32_RTIT_CR3_MATCH,
>  	MSR_IA32_RTIT_OUTPUT_BASE, MSR_IA32_RTIT_OUTPUT_MASK,
>  	MSR_IA32_RTIT_ADDR0_A, MSR_IA32_RTIT_ADDR0_B,
> -- 

Hmm, KVM shouldn't report the MSR if it can't be written by the guest.  Over-
reporting won't cause functional issues, and the odds of the MSR existing but not
being reported in ARCH_CAPILIBITES are basically zilch, but IMO it's worth adding
the check if only to document when the MSRs is fully supported.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e7f78fe79b32..d8608c6753ff 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7152,6 +7152,10 @@ static void kvm_probe_msr_to_save(u32 msr_index)
                if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
                        return;
                break;
+       case MSR_IA32_TSX_CTRL:
+               if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
+                       return;
+               break;
        default:
                break;
        }