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@xxxxxxxxxx>
Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
---
Changes:
v6:
* fail in LAUNCH_UPDATE_VMSA instead of clearing the flag
* pr_warn_ratelimited -> pr_warn_once
* due to the rework, removed Tom's "rb"
v5:
* new in the series
---
arch/x86/kvm/svm/sev.c | 5 +++++
arch/x86/kvm/svm/svm.c | 9 ++++++++-
2 files changed, 13 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 36fe2fcb4698..981286359b72 100644
--- 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 9c1b191aed4b..bec6fb82f494 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1996,7 +1996,7 @@ 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);
@@ -2727,6 +2727,13 @@ 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
--
2.40.1
