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| author | Paolo Bonzini <pbonzini@redhat.com> | 2018-08-22 19:59:33 +0200 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2018-10-17 00:30:02 +0200 |
| commit | 0e0a53c551317654e2d7885fdfd23299fee99b6b (patch) | |
| tree | e7c3d7c079231e99dacb2d303f44243b704fb3ea /arch/x86/kvm | |
| parent | f21dd494506ad002a5b6b32e50a5d4ccac6929fe (diff) | |
| download | lwn-0e0a53c551317654e2d7885fdfd23299fee99b6b.tar.gz lwn-0e0a53c551317654e2d7885fdfd23299fee99b6b.zip | |
kvm: x86: optimize dr6 restore
The quote from the comment almost says it all: we are currently zeroing
the guest dr6 in kvm_arch_vcpu_put, because do_debug expects it. However,
the host %dr6 is either:
- zero because the guest hasn't run after kvm_arch_vcpu_load
- written from vcpu->arch.dr6 by vcpu_enter_guest
- written by the guest and copied to vcpu->arch.dr6 by ->sync_dirty_debug_regs().
Therefore, we can skip the write if vcpu->arch.dr6 is already zero. We
may do extra useless writes if vcpu->arch.dr6 is nonzero but the guest
hasn't run; however that is less important for performance.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'arch/x86/kvm')
| -rw-r--r-- | arch/x86/kvm/x86.c | 13 |
1 files changed, 9 insertions, 4 deletions
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 20a667da0a31..92159db9cf71 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3186,11 +3186,16 @@ 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. + * Here dr6 is either zero or, if the guest has run and userspace + * has not set any breakpoints or watchpoints, it can be set to + * the guest dr6 (stored in vcpu->arch.dr6). do_debug expects dr6 + * to be cleared after it runs, so clear the host register. However, + * MOV to DR can be expensive when running nested, omit it if + * vcpu->arch.dr6 is already zero: in that case, the host dr6 cannot + * currently be nonzero. */ - set_debugreg(0, 6); + if (vcpu->arch.dr6) + set_debugreg(0, 6); } static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu, |