Re: [PATCH V2 3/3] KVM: X86: Reset DR6 only when KVM_DEBUGREG_WONT_EXIT

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/08/21 19:43, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> The commit efdab992813fb ("KVM: x86: fix escape of guest dr6 to the host")
> fixed a bug by reseting DR6 unconditionally when the vcpu being scheduled out.
> 
> But writing to debug registers is slow, and it can be shown in perf results
> sometimes even neither the host nor the guest activate breakpoints.
> 
> It'd be better to reset it conditionally and this patch moves the code of
> reseting DR6 to the path of VM-exit and only reset it when
> KVM_DEBUGREG_WONT_EXIT which is the only case that DR6 is guest value.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
>   arch/x86/kvm/x86.c | 8 ++------
>   1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d2aa49722064..f40cdd7687d8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4309,12 +4309,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>   
>   	static_call(kvm_x86_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.
> -	 */
> -	set_debugreg(0, 6);
>   }
>   
>   static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
> @@ -9630,6 +9624,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   		static_call(kvm_x86_sync_dirty_debug_regs)(vcpu);
>   		kvm_update_dr0123(vcpu);
>   		kvm_update_dr7(vcpu);
> +		/* Reset Dr6 which is guest value. */

Better keep the rationale from the original comment,

	/*
	 * do_debug expects dr6 to be cleared after it runs, so do
	 * not leave the guest value in the host DR6.
	 */

Paolo

> +		set_debugreg(DR6_RESERVED, 6);
>   	}
>   
>   	/*
>