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