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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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