Re: [kvm-unit-tests PATCH 02/16] x86: add few helper functions for apic local timer

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On Mon, 2022-10-24 at 16:10 +0000, Sean Christopherson wrote:
> On Mon, Oct 24, 2022, Maxim Levitsky wrote:
> > On Thu, 2022-10-20 at 19:14 +0000, Sean Christopherson wrote:
> > > On Thu, Oct 20, 2022, Maxim Levitsky wrote:
> > > > +       // ensure that a pending timer is serviced
> > > > +       irq_enable();
> > > 
> > > Jumping back to the "nop" patch, I'm reinforcing my vote to add sti_nop().  I
> > > actually starting typing a response to say this is broken before remembering that
> > > a nop got added to irq_enable().
> > 
> > OK, although, for someone that doesn't know about the interrupt shadow (I
> > guess most of the people that will look at this code), the above won't
> > confuse them, in fact sti_nop() might confuse someone who doesn't know about
> > why this nop is needed.
> 
> The difference is that sti_nop() might leave unfamiliar readers asking "why", but
> it won't actively mislead them.  And the "why" can be easily answered by a comment
> above sti_nop() to describe its purpose.  A "see also safe_halt()" with a comment
> there would be extra helpful, as "safe halt" is the main reason the STI shadow is
> even a thing.
> 
> On the other hand, shoving a NOP into irq_enable() is pretty much guaranteed to
> cause problems for readers that do know about STI shadows since there's nothing
> in the name "irq_enable" that suggests that the helper also intentionally eats the
> interrupt shadow, and especically because the kernel's local_irq_enable() distills
> down to a bare STI.

I still don't agree with you on this at all. I would like to hear what other KVM developers
think about it.

safe_halt actually is a example for function that abstacts away the nop - just what I want to do.

A comment in irq_enable() about that nop also is fine to have.

Best regards,
	Maxim Levitsky


> 





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