[PATCH kernel v5 3/6] KVM: SEV-ES: explicitly disable debug

[Alexey Kardashevskiy]

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;
- clear vcpu->guest_debug at SEV-ES' LAUNCH_UPDATE_VMSA if debugging
was already enabled; after that point the generic x86's
KVM_SET_GUEST_DEBUG ioctl disallows enabling debug.

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@xxxxxxxxxx>
Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
Changes:
v5:
* new in the series
---
 arch/x86/kvm/svm/sev.c |  6 ++++++
 arch/x86/kvm/svm/svm.c | 10 +++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 0f4761a57d86..b4365622222b 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -639,6 +639,12 @@ static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu,
 	  return ret;
 
 	vcpu->arch.guest_state_protected = true;
+
+	if (vcpu->guest_debug)
+		pr_warn_ratelimited("guest_debug (%lx) not supported for SEV-ES",
+				    vcpu->guest_debug);
+	vcpu->guest_debug = 0;
+
 	return 0;
 }
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index dc12de325cca..179952a31d3b 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1980,7 +1980,7 @@ 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);
@@ -2698,6 +2698,14 @@ 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
-- 
2.39.1