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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>
This commit is contained in:
Paolo Bonzini 2018-08-22 19:59:33 +02:00
parent f21dd49450
commit 0e0a53c551
1 changed files with 9 additions and 4 deletions
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13
arch/x86/kvm/x86.c
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@ -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,