KVM: SEV-ES: explicitly disable debug

SVM/SEV enable debug registers intercepts to skip swapping DRs
on entering/exiting the guest. When the guest is in control of
debug registers (vcpu->guest_debug == 0), there is an optimisation to
reduce the number of context switches: intercepts are cleared and
the KVM_DEBUGREG_WONT_EXIT flag is set to tell KVM to do swapping
on guest enter/exit.

The same code also executes for SEV-ES, however it has no effect as
- it always takes (vcpu->guest_debug == 0) branch;
- KVM_DEBUGREG_WONT_EXIT is set but DR7 intercept is not cleared;
- vcpu_enter_guest() writes DRs but VMRUN for SEV-ES swaps them
with the values from _encrypted_ VMSA.

Be explicit about SEV-ES not supporting debug:
- return right away from dr_interception() and skip unnecessary processing;
- return an error right away from the KVM_SEV_LAUNCH_UPDATE_VMSA handler
if debugging was already enabled.
KVM_SET_GUEST_DEBUG are failing already after KVM_SEV_LAUNCH_UPDATE_VMSA
is finished due to vcpu->arch.guest_state_protected set to true.

Add WARN_ON to kvm_x86::sync_dirty_debug_regs() (saves guest DRs on
guest exit) to signify that SEV-ES won't hit that path.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Link: https://lore.kernel.org/r/20230615063757.3039121-5-aik@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: aik

arch/x86/kvm/svm/sev.c

@@ -619,6 +619,11 @@ static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu,
 	struct vcpu_svm *svm = to_svm(vcpu);
 	int ret;
 
+	if (vcpu->guest_debug) {
+		pr_warn_once("KVM_SET_GUEST_DEBUG for SEV-ES guest is not supported");
+		return -EINVAL;
+	}
+
 	/* Perform some pre-encryption checks against the VMSA */
 	ret = sev_es_sync_vmsa(svm);
 	if (ret)

arch/x86/kvm/svm/svm.c

@@ -1983,7 +1983,7 @@ static void svm_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
-	if (vcpu->arch.guest_state_protected)
+	if (WARN_ON_ONCE(sev_es_guest(vcpu->kvm)))
 		return;
 
 	get_debugreg(vcpu->arch.db[0], 0);
@@ -2714,6 +2714,13 @@ static int dr_interception(struct kvm_vcpu *vcpu)
 	unsigned long val;
 	int err = 0;
 
+	/*
+	 * SEV-ES intercepts DR7 only to disable guest debugging and the guest issues a VMGEXIT
+	 * for DR7 write only. KVM cannot change DR7 (always swapped as type 'A') so return early.
+	 */
+	if (sev_es_guest(vcpu->kvm))
+		return 1;
+
 	if (vcpu->guest_debug == 0) {
 		/*
 		 * No more DR vmexits; force a reload of the debug registers