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

[Xiaoyao Li]

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;