Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT

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On 2021/8/10 18:07, Paolo Bonzini wrote:
On 09/08/21 19:43, Lai Jiangshan wrote:
From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug
registers") allows the guest accessing to DRs without exiting when
KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized
on entry to the guest---including DR6 that was not synced before the commit.

But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT,
but also when KVM_DEBUGREG_BP_ENABLED.  The second case is unnecessary
and just leads to a more case which leaks stale DR6 to the host which has
to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put().

We'd better to set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT,
so that we can fine-grain control the cases when we need to reset it
which is done in later patch.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
  arch/x86/kvm/x86.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ad47a09ce307..d2aa49722064 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9598,7 +9598,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
          set_debugreg(vcpu->arch.eff_db[1], 1);
          set_debugreg(vcpu->arch.eff_db[2], 2);
          set_debugreg(vcpu->arch.eff_db[3], 3);
-        set_debugreg(vcpu->arch.dr6, 6);
+        /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
+        if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
+            set_debugreg(vcpu->arch.dr6, 6);
      } else if (unlikely(hw_breakpoint_active())) {
          set_debugreg(0, 7);
      }


Even better, this should be moved to vmx.c's vcpu_enter_guest.  This
matches the handling in svm.c:

         /*
          * Run with all-zero DR6 unless needed, so that we can get the exact cause
          * of a #DB.
          */
         if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
                 svm_set_dr6(svm, vcpu->arch.dr6);
         else
                 svm_set_dr6(svm, DR6_ACTIVE_LOW);

That is,

     KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT
     Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug
     registers") allows the guest accessing to DRs without exiting when
     KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized
     on entry to the guest---including DR6 that was not synced before the commit.
     But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT,
     but also when KVM_DEBUGREG_BP_ENABLED.  The second case is unnecessary
     and just leads to a more case which leaks stale DR6 to the host which has
     to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put().
     Even if KVM_DEBUGREG_WONT_EXIT, however, setting the host DR6 only matters
     on VMX because SVM always uses the DR6 value from the VMCB.  So move this
     line to vmx.c and make it conditional on KVM_DEBUGREG_WONT_EXIT.
     Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ae8e62df16dd..21a3ef3012cf 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6625,6 +6625,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
          vmx->loaded_vmcs->host_state.cr4 = cr4;
      }

+    /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
+    if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
+        set_debugreg(vcpu->arch.dr6, 6);


I also noticed the related code in svm.c, but I refrained myself
to add a new branch in vmx_vcpu_run().  But after I see you put
the code of resetting dr6 in vmx_sync_dirty_debug_regs(), the whole
solution is much clean and better.

And if any chance you are also concern about the additional branch,
could you add a new callback to set dr6 and call the callback from
x86.c when KVM_DEBUGREG_WONT_EXIT.

The possible implementation of the callback:
for vmx: set_debugreg(vcpu->arch.dr6, 6);
for svm: svm_set_dr6(svm, vcpu->arch.dr6);
         and always do svm_set_dr6(svm, DR6_ACTIVE_LOW); at the end of the
         svm_handle_exit().

Thanks
Lai

+
      /* When single-stepping over STI and MOV SS, we must clear the
       * corresponding interruptibility bits in the guest state. Otherwise
       * vmentry fails as it then expects bit 14 (BS) in pending debug
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a111899ab2b4..fbc536b21585 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9597,7 +9597,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
          set_debugreg(vcpu->arch.eff_db[1], 1);
          set_debugreg(vcpu->arch.eff_db[2], 2);
          set_debugreg(vcpu->arch.eff_db[3], 3);
-        set_debugreg(vcpu->arch.dr6, 6);
      } else if (unlikely(hw_breakpoint_active())) {
          set_debugreg(0, 7);
      }

Paolo



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