Re: [PATCH v15 09/41] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

[Xiaoyao Li]

On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> Load the guest's FPU state if userspace is accessing MSRs whose values
> are managed by XSAVES. Introduce two helpers, kvm_{get,set}_xstate_msr(),
> to facilitate access to such kind of MSRs.
>
> If MSRs supported in kvm_caps.supported_xss are passed through to guest,
> the guest MSRs are swapped with host's before vCPU exits to userspace and
> after it reenters kernel before next VM-entry.
>
> Because the modified code is also used for the KVM_GET_MSRS device ioctl(),
> explicitly check @vcpu is non-null before attempting to load guest state.
> The XSAVE-managed MSRs cannot be retrieved via the device ioctl() without
> loading guest FPU state (which doesn't exist).
>
> Note that guest_cpuid_has() is not queried as host userspace is allowed to
> access MSRs that have not been exposed to the guest, e.g. it might do
> KVM_SET_MSRS prior to KVM_SET_CPUID2.
>
> The two helpers are put here in order to manifest accessing xsave-managed
> MSRs requires special check and handling to guarantee the correctness of
> read/write to the MSRs.
>
> Co-developed-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Tested-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> [sean: drop S_CET, add big comment, move accessors to x86.c]
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>