[PATCH v3 2/8] KVM: x86: Prioritize DR7.GD #DB over #GP due to illegal DR6/7 value

[Sean Christopherson <seanjc@google.com>]

When emulating a MOV DR, specifically a write to DR6 or DR7, treat a #DB
due to DR7.GD (General Detect) as higher priority than a #GP due to an
illegal value.  While neither Intel's SDM nor AMD's APM says anything
about the relative priority, empirical testing on Intel and AMD shows that
the #DB has higher priority.  And for VMX, where the instruction intercept
has priority over *all* exceptions, KVM already treats the #DB as having
higher priority.

Cc: Maciej W. Rozycki <macro@orcam.me.uk>
Fixes: 3b88e41a4134 ("KVM: SVM: Add intercept check for accessing dr registers")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/emulate.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 75cd8b6136aa..4484c5fa19e3 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -3854,11 +3854,16 @@ static int check_dr_write(struct x86_emulate_ctxt *ctxt)
 {
 	u64 new_val = ctxt->src.val64;
 	int dr = ctxt->modrm_reg;
+	int rc;
+
+	rc = check_dr_read(ctxt);
+	if (rc != X86EMUL_CONTINUE)
+		return rc;
 
 	if ((dr == 6 || dr == 7) && (new_val & 0xffffffff00000000ULL))
 		return emulate_gp(ctxt, 0);
 
-	return check_dr_read(ctxt);
+	return X86EMUL_CONTINUE;
 }
 
 static int check_svme(struct x86_emulate_ctxt *ctxt)
-- 
2.54.0.1136.gdb2ca164c4-goog