[PATCH v3 5/8] KVM: VMX: Prioritize DR7.GD=1 #DB over CPL>0 #GP on Intel

[Sean Christopherson <seanjc@google.com>]

When emulating a MOV DR on Intel with DR7.GD=1 at CPL>0, prioritize the #DB
due to DR7.GD over the #GP due to CPL>0, as empirical testing shows that
Intel CPUs (Skylake, Icelake and Emerald Rapids) prioritize the DR7.GD #DB
over all #GPs, whereas AMD CPUs prioritize the CPL>0 #GP (but not illegal
value #GPs) over the #DB.

Outside of the emulator, don't bother trying to provide the "correct"
priority based on the virtual CPU model, as it's simply impossible to do
so without intercepting *all* MOV DR accesses, which would result in a
massive, unacceptable performance hit.  Note, getting the priority right
when advertising Intel on AMD would also require intercepting #GP, as SVM
prioritizes all exceptions over the instruction intercept.

Note, neither Intel's SDM nor AMD's APM says anything about the relative
priority, hence the empirical testing.  Arguably Intel's description of
DR7.GD:

  causes a debug exception to be generated prior to any MOV instruction
  that accesses a debug register.

implies that DR7.GD has higher priority.  But that's a fairly weak argument
as the statement would still hold true if the #GP due to CPL>0 had higher
priority, as the #GP would prevent any access to a DR.

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 ++++++-
 arch/x86/kvm/vmx/vmx.c | 6 +++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 127a21eeef66..b4dc57fe0bc9 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -3834,6 +3834,7 @@ static int check_cr_access(struct x86_emulate_ctxt *ctxt)
 
 static int check_dr_read(struct x86_emulate_ctxt *ctxt)
 {
+	bool is_intel = ctxt->ops->guest_cpuid_is_intel_compatible(ctxt);
 	int dr = ctxt->modrm_reg;
 	u64 cr4;
 
@@ -3844,12 +3845,16 @@ static int check_dr_read(struct x86_emulate_ctxt *ctxt)
 	if ((cr4 & X86_CR4_DE) && (dr == 4 || dr == 5))
 		return emulate_ud(ctxt);
 
-	if (ctxt->ops->cpl(ctxt))
+	/* Intel CPUs prioritize the DR7.GD=1 #DB over the CPL>0 #GP. */
+	if (!is_intel && ctxt->ops->cpl(ctxt))
 		return emulate_gp(ctxt, 0);
 
 	if (ctxt->ops->get_effective_dr7(ctxt) & DR7_GD)
 		return emulate_db(ctxt, DR6_BD);
 
+	if (is_intel && ctxt->ops->cpl(ctxt))
+		return emulate_gp(ctxt, 0);
+
 	return X86EMUL_CONTINUE;
 }
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c548f22375ad..2f13d3163367 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5750,9 +5750,6 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 	if (!kvm_require_dr(vcpu, dr))
 		return 1;
 
-	if (vmx_get_cpl(vcpu) > 0)
-		goto out;
-
 	dr7 = vmcs_readl(GUEST_DR7);
 	if (dr7 & DR7_GD) {
 		/*
@@ -5773,6 +5770,9 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 		}
 	}
 
+	if (vmx_get_cpl(vcpu) > 0)
+		goto out;
+
 	if (vcpu->guest_debug == 0) {
 		exec_controls_clearbit(to_vmx(vcpu), CPU_BASED_MOV_DR_EXITING);
 
-- 
2.54.0.1136.gdb2ca164c4-goog