[PATCH 3/3] KVM: VMX: Extend VMX's #AC interceptor to handle split lock #AC in guest

[Sean Christopherson <sean.j.christopherson@intel.com>]

From: Xiaoyao Li <xiaoyao.li@intel.com>

Two types #AC can be generated in Intel CPUs:
 1. legacy alignment check #AC
 2. split lock #AC

Reflect #AC back into the guest if the guest has legacy alignment checks
enabled or if SLD is disabled.  If SLD is enabled, treat the guest like
a host userspace application by calling handle_user_split_lock().  If
the #AC is handled (SLD disabled and TIF_SLD set), then simply resume
the guest.  If the #AC isn't handled, i.e. host is sld_fatal, then
forward the #AC to the userspace VMM, similar to sending SIGBUS.

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/kvm/vmx/vmx.c | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 458e684dfbdc..a96cfda0a5b9 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4623,6 +4623,12 @@ static int handle_machine_check(struct kvm_vcpu *vcpu)
 	return 1;
 }
 
+static inline bool guest_cpu_alignment_check_enabled(struct kvm_vcpu *vcpu)
+{
+	return vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
+	       (kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
+}
+
 static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -4688,9 +4694,6 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 		return handle_rmode_exception(vcpu, ex_no, error_code);
 
 	switch (ex_no) {
-	case AC_VECTOR:
-		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
-		return 1;
 	case DB_VECTOR:
 		dr6 = vmcs_readl(EXIT_QUALIFICATION);
 		if (!(vcpu->guest_debug &
@@ -4719,6 +4722,27 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
 		kvm_run->debug.arch.exception = ex_no;
 		break;
+	case AC_VECTOR:
+		/*
+		 * Reflect #AC to the guest if it's expecting the #AC, i.e. has
+		 * legacy alignment check enabled.  Pre-check host split lock
+		 * turned on to avoid the VMREADs needed to check legacy #AC,
+		 * i.e. reflect the #AC if the only possible source is legacy
+		 * alignment checks.
+		 */
+		if (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) ||
+		    guest_cpu_alignment_check_enabled(vcpu)) {
+			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+			return 1;
+		}
+
+		/*
+		 * Forward the #AC to userspace if kernel policy does not allow
+		 * temporarily disabling split lock detection.
+		 */
+		if (handle_user_split_lock(kvm_rip_read(vcpu)))
+			return 1;
+		fallthrough;
 	default:
 		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
 		kvm_run->ex.exception = ex_no;
-- 
2.24.1