[PATCH v5 6/9] kvm: vmx: Extend VMX's #AC interceptor to handle split lock #AC happens in guest

From: Xiaoyao Li <xiaoyao.li@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	hpa@zytor.com, Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	kvm@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org
Cc: Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jim Mattson <jmattson@google.com>,
	Xiaoyao Li <xiaoyao.li@intel.com>
Subject: [PATCH v5 6/9] kvm: vmx: Extend VMX's #AC interceptor to handle split lock #AC happens in guest
Date: Sun, 15 Mar 2020 13:05:14 +0800	[thread overview]
Message-ID: <20200315050517.127446-7-xiaoyao.li@intel.com> (raw)
In-Reply-To: <20200315050517.127446-1-xiaoyao.li@intel.com>

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., sld_warn or sld_fatal,
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@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@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 40b1e6138cd5..3fb132ad489d 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4609,6 +4609,12 @@ 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);
@@ -4674,9 +4680,6 @@ 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 &
@@ -4705,6 +4708,27 @@ 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
+		 * support to avoid the VMREADs needed to check legacy #AC,
+		 * i.e. reflect the #AC if the only possible source is legacy
+		 * alignment checks.
+		 */
+		if (!split_lock_detect_on() ||
+		    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;
-- 
2.20.1

