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

[Xiaoyao Li]

On 10/15/2021 3:28 PM, Hao Xiang wrote:
> 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.
>   */

(I'm not sure if forcely is a valid word)

how about

/*
 * Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
 * VM Exit. Unconditionally set the flag here and leave the handling to
 * vmx_handle_exit().
 */

> > > +     */
> > > +    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)