Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted

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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?


Thanks,

Paolo

Signed-off-by: Hao Xiang <hao.xiang@xxxxxxxxxxxxxxxxx>
---
  arch/x86/kvm/vmx/vmx.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0c2c0d5..5ddf1df 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6054,7 +6054,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)        * still need to 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 (to_vmx(vcpu)->exit_reason.bus_lock_detected &&
+            to_vmx(vcpu)->exit_reason.basic != EXIT_REASON_BUS_LOCK) {
          if (ret > 0)
              vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;






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