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KVM: x86: Call out MSR_IA32_S_CET is not handled by XSAVES

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Update the comment above is_xstate_managed_msr() to note that
MSR_IA32_S_CET isn't saved/restored by XSAVES/XRSTORS.

MSR_IA32_S_CET isn't part of CET_U/S state as the SDM states:
  The register state used by Control-Flow Enforcement Technology (CET)
  comprises the two 64-bit MSRs (IA32_U_CET and IA32_PL3_SSP) that manage
  CET when CPL = 3 (CET_U state); and the three 64-bit MSRs
  (IA32_PL0_SSP–IA32_PL2_SSP) that manage CET when CPL < 3 (CET_S state).

Opportunistically shift the snippet about the safety of loading certain
MSRs to the function comment for kvm_access_xstate_msr(), which is where
the MSRs are actually loaded into hardware.

Fixes: e44eb58334 ("KVM: x86: Load guest FPU state when access XSAVE-managed MSRs")
Signed-off-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20251028060142.29830-1-chao.gao@intel.com
[sean: shift snippet about safety to kvm_access_xstate_msr()]
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Chao Gao
2025-10-27 23:01:41 -07:00
committed by Sean Christopherson
parent 9bc610b6a2
commit cab4098be4
1 changed files with 8 additions and 9 deletions
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17
arch/x86/kvm/x86.c
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@@ -3874,15 +3874,9 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
/* /*
* Returns true if the MSR in question is managed via XSTATE, i.e. is context * Returns true if the MSR in question is managed via XSTATE, i.e. is context
* switched with the rest of guest FPU state. Note! S_CET is _not_ context * switched with the rest of guest FPU state.
* switched via XSTATE even though it _is_ saved/restored via XSAVES/XRSTORS. *
* Because S_CET is loaded on VM-Enter and VM-Exit via dedicated VMCS fields, * Note, S_CET is _not_ saved/restored via XSAVES/XRSTORS.
* the value saved/restored via XSTATE is always the host's value. That detail
* is _extremely_ important, as the guest's S_CET must _never_ be resident in
* hardware while executing in the host. Loading guest values for U_CET and
* PL[0-3]_SSP while executing in the kernel is safe, as U_CET is specific to
* userspace, and PL[0-3]_SSP are only consumed when transitioning to lower
* privilege levels, i.e. are effectively only consumed by userspace as well.
*/ */
static bool is_xstate_managed_msr(struct kvm_vcpu *vcpu, u32 msr) static bool is_xstate_managed_msr(struct kvm_vcpu *vcpu, u32 msr)
{ {
@@ -3905,6 +3899,11 @@ static bool is_xstate_managed_msr(struct kvm_vcpu *vcpu, u32 msr)
* MSR that is managed via XSTATE. Note, the caller is responsible for doing * MSR that is managed via XSTATE. Note, the caller is responsible for doing
* the initial FPU load, this helper only ensures that guest state is resident * the initial FPU load, this helper only ensures that guest state is resident
* in hardware (the kernel can load its FPU state in IRQ context). * in hardware (the kernel can load its FPU state in IRQ context).
*
* Note, loading guest values for U_CET and PL[0-3]_SSP while executing in the
* kernel is safe, as U_CET is specific to userspace, and PL[0-3]_SSP are only
* consumed when transitioning to lower privilege levels, i.e. are effectively
* only consumed by userspace as well.
*/ */
static __always_inline void kvm_access_xstate_msr(struct kvm_vcpu *vcpu, static __always_inline void kvm_access_xstate_msr(struct kvm_vcpu *vcpu,
struct msr_data *msr_info, struct msr_data *msr_info,