Re: [PATCH v2] KVM: VMX: Remove redundant handling of bus lock vmexit

[Hao Xiang]

On 2021/10/15 14:46, Xiaoyao Li wrote:
> On 10/15/2021 11:57 AM, Hao Xiang wrote:
> > Hardware may or may not set exit_reason.bus_lock_detected on BUS_LOCK
> > VM-Exits. Dealing with KVM_RUN_X86_BUS_LOCK in handle_bus_lock_vmexit
> > could be redundant when exit_reason.basic is EXIT_REASON_BUS_LOCK.
> >
> > We can remove redundant handling of bus lock vmexit. Force Setting
> > exit_reason.bus_lock_detected in handle_bus_lock_vmexit(), and deal with
> > KVM_RUN_X86_BUS_LOCK only in vmx_handle_exit().
> >
> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Hao Xiang <hao.xiang@xxxxxxxxxxxxxxxxx>
> > ---
> > v1 -> v2: a little modifications of comments
> >
> >   arch/x86/kvm/vmx/vmx.c | 15 +++++++++------
> >   1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 116b089..22be02e 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -5562,9 +5562,13 @@ 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;
> > +    /*
> > +     * Hardware may or may not set the BUS_LOCK_DETECTED flag on 
> > BUS_LOCK
> > +     * VM-Exits, force setting the flag 
>
> >          so that the logic in vmx_handle_exit()
> > +     * doesn't have to handle the flag and the basic exit reason.
>
> The code itself looks good.
>
> But the comment is partly incorrect. Now we rely only on the 
> bus_lock_detected flag so vmx_handle_exit() must check the flag.
> Thank you for you advice. Is the following comment ok?
/*
 * Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
 * VM-Exits. Forcely set the flag here, then check the flag in
 * vmx_handle_exit() and handle the bus lock vmexit if the flag is
 * set.
 */

> > +     */
> > +    to_vmx(vcpu)->exit_reason.bus_lock_detected = true;
> > +    return 1;
> >   }
> >   /*
> > @@ -6051,9 +6055,8 @@ 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)