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>

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index d58a25a..0299bbe 100644 (file)
--- a/arch/x86/kvm/svm/sev.c
+++ b/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)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index a0e803f..e838cfe 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/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