KVM: SEV: Synchronize MSR_IA32_XSS from the GHCB when it's valid

Synchronize XSS from the GHCB to KVM's internal tracking if the guest
marks XSS as valid on a #VMGEXIT.  Like XCR0, KVM needs an up-to-date copy
of XSS in order to compute the required XSTATE size when emulating
CPUID.0xD.0x1 for the guest.

Treat the incoming XSS change as an emulated write, i.e. validatate the
guest-provided value, to avoid letting the guest load garbage into KVM's
tracking.  Simply ignore bad values, as either the guest managed to get an
unsupported value into hardware, or the guest is misbehaving and providing
pure garbage.  In either case, KVM can't fix the broken guest.

Explicitly allow access to XSS at all times, as KVM needs to ensure its
copy of XSS stays up-to-date.  E.g. KVM supports migration of SEV-ES guests
and so needs to allow the host to save/restore XSS, otherwise a guest
that *knows* its XSS hasn't change could get stale/bad CPUID emulation if
the guest doesn't provide XSS in the GHCB on every exit.  This creates a
hypothetical problem where a guest could request emulation of RDMSR or
WRMSR on XSS, but arguably that's not even a problem, e.g. it would be
entirely reasonable for a guest to request "emulation" as a way to inform
the hypervisor that its XSS value has been modified.

Note, emulating the change as an MSR write also takes care of side effects,
e.g. marking dynamic CPUID bits as dirty.

Suggested-by: John Allen <john.allen@amd.com>
base-commit: 14298d819d5a6b7180a4089e7d2121ca3551dc6c
Link: https://lore.kernel.org/r/20250919223258.1604852-40-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/sev.c

@@ -3310,6 +3310,9 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
 	if (kvm_ghcb_xcr0_is_valid(svm))
 		__kvm_set_xcr(vcpu, 0, kvm_ghcb_get_xcr0(svm));
 
+	if (kvm_ghcb_xss_is_valid(svm))
+		__kvm_emulate_msr_write(vcpu, MSR_IA32_XSS, kvm_ghcb_get_xss(svm));
+
 	/* Copy the GHCB exit information into the VMCB fields */
 	exit_code = kvm_ghcb_get_sw_exit_code(svm);
 	control->exit_code = lower_32_bits(exit_code);

arch/x86/kvm/svm/svm.c

@@ -2721,8 +2721,8 @@ static int svm_get_feature_msr(u32 msr, u64 *data)
 static bool sev_es_prevent_msr_access(struct kvm_vcpu *vcpu,
 				      struct msr_data *msr_info)
 {
-	return sev_es_guest(vcpu->kvm) &&
-	       vcpu->arch.guest_state_protected &&
+	return sev_es_guest(vcpu->kvm) && vcpu->arch.guest_state_protected &&
+	       msr_info->index != MSR_IA32_XSS &&
 	       !msr_write_intercepted(vcpu, msr_info->index);
 }
 

arch/x86/kvm/svm/svm.h

@@ -941,5 +941,6 @@ DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_1)
 DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_2)
 DEFINE_KVM_GHCB_ACCESSORS(sw_scratch)
 DEFINE_KVM_GHCB_ACCESSORS(xcr0)
+DEFINE_KVM_GHCB_ACCESSORS(xss)
 
 #endif