Re: [PATCH v15 05/41] KVM: x86: Report XSS as to-be-saved if there are supported features

Binbin Wu
2025-09-16T07:12:42Z

On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> Add MSR_IA32_XSS to list of MSRs reported to userspace if supported_xss
> is non-zero, i.e. KVM supports at least one XSS based feature.
>
> Before enabling CET virtualization series, guest IA32_MSR_XSS is
> guaranteed to be 0, i.e., XSAVES/XRSTORS is executed in non-root mode
> with XSS == 0, which equals to the effect of XSAVE/XRSTOR.
>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> Reviewed-by: Chao Gao <chao.gao@intel.com>
> Tested-by: Mathias Krause <minipli@grsecurity.net>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
> arch/x86/kvm/x86.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 771b7c883c66..3b4258b38ad8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -332,7 +332,7 @@ static const u32 msrs_to_save_base[] = {
> MSR_IA32_RTIT_ADDR3_A, MSR_IA32_RTIT_ADDR3_B,
> MSR_IA32_UMWAIT_CONTROL,
>
> - MSR_IA32_XFD, MSR_IA32_XFD_ERR,
> + MSR_IA32_XFD, MSR_IA32_XFD_ERR, MSR_IA32_XSS,
> };
>
> static const u32 msrs_to_save_pmu[] = {
> @@ -7499,6 +7499,10 @@ static void kvm_probe_msr_to_save(u32 msr_index)
> if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
> return;
> break;
> + case MSR_IA32_XSS:
> + if (!kvm_caps.supported_xss)
> + return;
> + break;
> default:
> break;
> }