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

Xiaoyao Li
2020-02-06T07:09:53Z

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@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
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