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

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 27, 2022, Maxim Levitsky wrote:
> 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.

Why not just kill off irq_enable() and irq_disable() and use sti() and cli()?
Then we don't have to come to any agreement on whether or not shoving a NOP into
irq_enable() is a good idea.

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

The difference is that "safe halt" is established terminology that specifically
means "STI immediately followed by HLT".