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

Paolo Bonzini
2021-08-10T10:14:45Z

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

... and this should also be done exclusively for VMX, in vmx_sync_dirty_debug_regs:

KVM: VMX: Reset DR6 only when KVM_DEBUGREG_WONT_EXIT

The commit efdab992813fb ("KVM: x86: fix escape of guest dr6 to the host")
fixed a bug by resetting DR6 unconditionally when the vcpu being scheduled out.

But writing to debug registers is slow, and it can be visible in perf results
sometimes, even if neither the host nor the guest activate breakpoints.

Since KVM_DEBUGREG_WONT_EXIT on Intel processors is the only case
where DR6 gets the guest value, and it never happens at all on SVM,
the register can be cleared in vmx.c right after reading it.

Reported-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 21a3ef3012cf..3a91302d05c0 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5110,6 +5110,12 @@ static void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)

vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_WONT_EXIT;
exec_controls_setbit(to_vmx(vcpu), CPU_BASED_MOV_DR_EXITING);
+
+ /*
+ * do_debug expects dr6 to be cleared after it runs, avoid that it sees
+ * a stale dr6 from the guest.
+ */
+ set_debugreg(DR6_RESERVED, 6);
}

static void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fbc536b21585..04c393551fb0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4313,12 +4313,6 @@ 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,

Paolo