Hi,
while debugging a nested VM setup where Linux runs as an L1
guest, I found lots of VM exits due to %dr6 writes. However,
neither L0 nor L1 or L2 where using debug registers.
The behaviour was introdued with change efdab992 and the
original author suggested the patch below.
BTW: I'm not related to the other guy with the same name.
regards Christian
>From e18794eaa15a6bd8829a86a806bc8acb964eefb9 Mon Sep 17 00:00:00 2001
From: Christian Ehrhardt <lk@xxxxxx>
Date: Mon, 15 Oct 2018 20:05:48 +0200
Subject: [PATCH 1/2] KVM: x86: Only clear %dr6 if really neccessary
Change efdab992: "KVM: x86: fix escape of guest dr6 to the host"
has negative effects if Linux runs as the L1 guest in a nested
VM setup:
Each call to kvm_arch_vcpu_put causes an additional L1->L0 exit due
to the %dr6 write even if neither L1 nor L2 uses debug registers.
As the current hardware value of %dr6 is already in vcpu->arch.dr6
we can avoid the debug register write if the value is already zero.
Update the comment and reformat it while there.
Signed-off-by: Christian Ehrhardt <lk@xxxxxxx>
---
arch/x86/kvm/x86.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ca717737347e..37d046c5c9c1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3185,11 +3185,13 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
kvm_x86_ops->vcpu_put(vcpu);
vcpu->arch.last_host_tsc = rdtsc();
/*
- * If userspace has set any breakpoints or watchpoints, dr6 is restored
- * on every vmexit, but if not, we might have a stale dr6 from the
- * guest. do_debug expects dr6 to be cleared after it runs, do the same.
+ * If userspace has set any breakpoints or watchpoints, dr6 is
+ * restored on every vmexit, but if not, we might have a stale
+ * dr6 from the guest. do_debug expects dr6 to be cleared after
+ * it runs, do the same if dr6 has a non-zero value.
*/
- set_debugreg(0, 6);
+ if (vcpu->arch.dr6)
+ set_debugreg(0, 6);
}
static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
--
2.17.1
