Re: [Xen-devel] vcpu_block() and do_poll() question

[Julien Grall <julien.grall@xxxxxxx>]

On 30/04/2019 11:27, Jan Beulich wrote:
> > > > On 30.04.19 at 11:13, <jgross@xxxxxxxx> wrote:
> > In xen/common/schedule.c there is a weird "#ifndef CONFIG_X86" in
> > do_poll().
> >
> > It was introduced way before anyone would think about ARM by commit
> > ef4c6b079cc55e (I couldn't find any xen-devel mail related to that
> > commit), so I guess it is related to IA64?
> Quite certainly, yes (or PPC as an alternative, but I know nothing
> about their memory ordering model).
>
> > Question is: can we just drop it, or does ARM depend on it? And if ARM
> > really needs it, is it the memory barrier only? And why wouldn't a
> > similar barrier be needed in vcpu_block() then?
> It's not the memory barrier, but the subsequent checks. The barrier
> is there only to make sure the checks don't happen before the
> earlier writes. And as the comment says - it's the combination of
> the 3 writes that may get mis-ordered and hence mis-interpreted by
> other (reading) parties. vcpu_block(), otoh, sets the VPF_blocked
> bit only, so there's no potential ordering issue there.
Are you suggesting the comment in vcpu_block is not correct?

Cheers,

--
Julien Grall

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