On Tue, 16 Oct 2018 at 04:45, Christian Ehrhardt <lk@xxxxxxx> wrote: > > > 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); I tried this immediately after the offline discussion between Paolo and you several months ago, however, I can observe the issue was still present. Regards, Wanpeng Li
