KVM: SVM: Rewrite sev_es_prepare_switch_to_guest()'s comment about swap types

Rewrite the comment(s) in sev_es_prepare_switch_to_guest() to explain the
swap types employed by the CPU for SEV-ES guests, i.e. to explain why KVM
needs to save a seemingly random subset of host state, and to provide a
decoder for the APM's Type-A/B/C terminology.

Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Link: https://lore.kernel.org/r/20230615063757.3039121-4-aik@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index fd6e316..d58a25a 100644 (file)
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3023,19 +3023,24 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm)
 void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
 {
        /*
-        * As an SEV-ES guest, hardware will restore the host state on VMEXIT,
-        * of which one step is to perform a VMLOAD.  KVM performs the
-        * corresponding VMSAVE in svm_prepare_guest_switch for both
-        * traditional and SEV-ES guests.
+        * All host state for SEV-ES guests is categorized into three swap types
+        * based on how it is handled by hardware during a world switch:
+        *
+        * A: VMRUN:   Host state saved in host save area
+        *    VMEXIT:  Host state loaded from host save area
+        *
+        * B: VMRUN:   Host state _NOT_ saved in host save area
+        *    VMEXIT:  Host state loaded from host save area
+        *
+        * C: VMRUN:   Host state _NOT_ saved in host save area
+        *    VMEXIT:  Host state initialized to default(reset) values
+        *
+        * Manually save type-B state, i.e. state that is loaded by VMEXIT but
+        * isn't saved by VMRUN, that isn't already saved by VMSAVE (performed
+        * by common SVM code).
         */
-
-       /* XCR0 is restored on VMEXIT, save the current host value */
        hostsa->xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
-
-       /* PKRU is restored on VMEXIT, save the current host value */
        hostsa->pkru = read_pkru();
-
-       /* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */
        hostsa->xss = host_xss;
 }