[RFC PATCH v2 4/6] KVM: SVM: Save shadow stack host state on VMRUN

[John Allen <john.allen@amd.com>]

When running as an SEV-ES guest, the PL0_SSP, PL1_SSP, PL2_SSP, PL3_SSP,
and U_CET fields in the VMCB save area are type B, meaning the host
state is automatically loaded on a VMEXIT, but is not saved on a VMRUN.
The other shadow stack MSRs, S_CET, SSP, and ISST_ADDR are type A,
meaning they are loaded on VMEXIT and saved on VMRUN. Manually save the
type B host MSR values before VMRUN.

Signed-off-by: John Allen <john.allen@amd.com>
---
 arch/x86/kvm/svm/sev.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index c25aeb550cd9..03dd68bddd51 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3028,6 +3028,19 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
 
 	/* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */
 	hostsa->xss = host_xss;
+
+	if (boot_cpu_has(X86_FEATURE_SHSTK)) {
+		/*
+		 * MSR_IA32_U_CET, MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP,
+		 * MSR_IA32_PL2_SSP, and MSR_IA32_PL3_SSP are restored on
+		 * VMEXIT, save the current host values.
+		 */
+		rdmsrl(MSR_IA32_U_CET, hostsa->u_cet);
+		rdmsrl(MSR_IA32_PL0_SSP, hostsa->vmpl0_ssp);
+		rdmsrl(MSR_IA32_PL1_SSP, hostsa->vmpl1_ssp);
+		rdmsrl(MSR_IA32_PL2_SSP, hostsa->vmpl2_ssp);
+		rdmsrl(MSR_IA32_PL3_SSP, hostsa->vmpl3_ssp);
+	}
 }
 
 void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
-- 
2.39.1