[PATCH v3 6/8] kvm: vmx: Extend VMX's #AC interceptor to handle split lock #AC happens in guest

[Xiaoyao Li]

There are two types of #AC can be generated in Intel CPUs:
 1. legacy alignment check #AC;
 2. split lock #AC;

Legacy alignment check #AC can be injected to guest if guest has enabled
alignemnet check.

when host enables split lock detectin, i.e., split_lock_detect != off,
there will be an unexpected #AC in guest and intercepted by KVM because
KVM doesn't virtualize this feature to guest and hardware value of
MSR_TEST_CTRL.SLD bit stays unchanged when vcpu is running.

To handle this unexpected #AC, treat guest just like host usermode that
calling handle_user_split_lock():
 - If host is sld_warn, it warns and set TIF_SLD so that __switch_to_xtra()
   does the MSR_TEST_CTRL.SLD bit switching when control transfer to/from
   this vcpu.
 - If host is sld_fatal, forward #AC to userspace, the similar as sending
   SIGBUS.

Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
v3:
 - Use handle_user_split_lock() to handle unexpected #AC in guest.
---
 arch/x86/kvm/vmx/vmx.c | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c475fa2aaae0..822211975e6c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4557,6 +4557,12 @@ 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);
@@ -4622,9 +4628,6 @@ 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 &
@@ -4653,6 +4656,28 @@ 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:
+		/*
+		 * Inject #AC back to guest only when guest enables legacy
+		 * alignment check.
+		 * Otherwise, it must be an unexpected split lock #AC of guest
+		 * since hardware SPLIT_LOCK_DETECT bit keeps unchanged set
+		 * when vcpu is running. In this case, treat guest the same as
+		 * user space application that calls handle_user_split_lock():
+		 *  - If sld_state = sld_warn, it sets TIF_SLD and disables SLD
+		 *    for this vcpu thread.
+		 *  - If sld_state = sld_fatal, we forward #AC to userspace,
+		 *    similar as sending SIGBUS.
+		 */
+		if (!split_lock_detect_enabled() ||
+		    guest_cpu_alignment_check_enabled(vcpu)) {
+			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+			return 1;
+		}
+
+		if (handle_user_split_lock(kvm_rip_read(vcpu)))
+			return 1;
+		/* fall through */
 	default:
 		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
 		kvm_run->ex.exception = ex_no;
-- 
2.23.0