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

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 30/04/2019 12:48, Jan Beulich wrote:
> > > > On 30.04.19 at 12:44, <jgross@xxxxxxxx> wrote:
> > An alternative would be memory barriers between the writes on ARM,
> > right? Or a strong ordered set_bit() variant (we had that discussion
> > recently related to a barrier in ARM-specific __cpu_disable()).
I am not entirely a big fan of a strong-order variant. It will potentially add 
more memory barriers than necessary in this context.
> Yes.
What would be the advantage of 2-3 memory barriers over a memory barrier + 
check?

> > Then we could drop this #ifndef section.
> Not sure about this - I'm actually unconvinced the latter part of
> what's inside the #ifdef isn't actually needed on x86 as well. Just
> consider the case of vcpu_unblock() making it to the vcpu_wake()
> invocation on another CPU while we're between any two of the
> three writes here. (I know I've been feeling uneasy about this
> before, but I guess I must have come to the conclusion that it's
> _probably_ okay.)
That's not going to be covered by the check on non-x86 platform. Indeed, 
vcpu_wake() is not updating any of the fields. So, from my understanding, the 
wake-up request will just be ignored.
Cheers,

--
Julien Grall

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