On 3/26/21 10:32 AM, Paolo Bonzini wrote:
On 23/03/21 18:50, Krish Sadhukhan wrote:
According to APM, the #DB intercept for a single-stepped VMRUN must
happen
after the completion of that instruction, when the guest does #VMEXIT to
the host. However, in the current implementation of KVM, the #DB
intercept
for a single-stepped VMRUN happens after the completion of the
instruction
that follows the VMRUN instruction. When the #DB intercept handler is
invoked, it shows the RIP of the instruction that follows VMRUN,
instead of
of VMRUN itself. This is an incorrect RIP as far as single-stepping
VMRUN
is concerned.
This patch fixes the problem by checking, in nested_svm_vmexit(), for
the
condition that the VMRUN instruction is being single-stepped and if so,
queues the pending #DB intercept so that the #DB is accounted for before
we execute L1's next instruction.
Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxxx>
---
arch/x86/kvm/svm/nested.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 35891d9a1099..713ce5cfb0db 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -720,6 +720,16 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
kvm_clear_exception_queue(&svm->vcpu);
kvm_clear_interrupt_queue(&svm->vcpu);
+ /*
+ * If we are here following the completion of a VMRUN that
+ * is being single-stepped, queue the pending #DB intercept
+ * right now so that it an be accounted for before we execute
+ * L1's next instruction.
+ */
+ if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_VMRUN &&
+ svm->vmcb->save.rflags & X86_EFLAGS_TF))
+ kvm_queue_exception(&(svm->vcpu), DB_VECTOR);
+
return 0;
}
Wouldn't the exit code always be SVM_EXIT_VMRUN after the
vmcb01/vmcb02 split? I can take care of adding a WARN_ON myself, but
I wouldn't mind if you checked that my reasoning is true. :)
Looking at the patchset on "vmcb01/vmcb02 split", I see that we are calling
svm_switch_vmcb(svm, &svm->vmcb01)
in nested_svm_vmexiit(). So, yes, by the time we reach the above check
we should always have an exit code of SVM_EXIT_VMRUN.
Paolo
