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

[Sean Christopherson <seanjc@google.com>]

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.