On 2021/9/22 22:58, Sean Christopherson wrote:
On Wed, Sep 22, 2021, Xiaoyao Li wrote:
On 9/22/2021 6:02 PM, Paolo Bonzini wrote:
On 18/09/21 13:30, Hao Xiang wrote:
exit_reason.bus_lock_detected is not only set when bus lock VM exit
was preempted, in fact, this bit is always set if bus locks are
detected no matter what the exit_reason.basic is.
So the bus_lock_vmexit handling in vmx_handle_exit should be duplicated
when exit_reason.basic is EXIT_REASON_BUS_LOCK(74). We can avoid it by
checking if bus lock vmexit was preempted in vmx_handle_exit.
I don't understand, does this mean that bus_lock_detected=1 if
basic=EXIT_REASON_BUS_LOCK? If so, can we instead replace the contents
of handle_bus_lock_vmexit with
/* Do nothing and let vmx_handle_exit exit to userspace. */
WARN_ON(!to_vmx(vcpu)->exit_reason.bus_lock_detected);
return 0;
?
That would be doable only if this is architectural behavior and not a
processor erratum, of course.
EXIT_REASON.bus_lock_detected may or may not be set when exit reason ==
EXIT_REASON_BUS_LOCK. Intel will update ISE or SDM to state it.
Maybe we can do below in handle_bus_lock_vmexit handler:
if (!to_vmx(vcpu)->exit_reason.bus_lock_detected)
to_vmx(vcpu)->exit_reason.bus_lock_detected = 1;
But is manually changing the hardware reported value for software purpose a
good thing?
In this case, I'd say yes. Hardware having non-deterministic behavior is the not
good thing, KVM would simply be correctly the not-technically-an-erratum erratum.
Set it unconditionally and then handle everything in common path. This has the
added advantage of having only one site that deals with KVM_RUN_X86_BUS_LOCK.
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 33f92febe3ce..aa9372452e49 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5561,9 +5561,9 @@ static int handle_encls(struct kvm_vcpu *vcpu)
static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
{
- vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
- vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
- return 0;
+ /* The dedicated flag may or may not be set by hardware. /facepalm. */
+ vcpu->exit_reason.bus_lock_detected = true;
+ return 1;
}
/*
@@ -6050,9 +6050,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
int ret = __vmx_handle_exit(vcpu, exit_fastpath);
/*
- * Even when current exit reason is handled by KVM internally, we
- * still need to exit to user space when bus lock detected to inform
- * that there is a bus lock in guest.
+ * Exit to user space when bus lock detected to inform that there is a
+ * bus lock in guest.
*/
if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
if (ret > 0)
I agree with your modifications. And I will re-submit the patch. Thanks.
