Re: [Xen-devel] [PATCH for-4.12 v2 17/17] xen/arm: Track page accessed between batch of Set/Way operations

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 05/12/2018 08:37, Jan Beulich wrote:
> > > > On 04.12.18 at 21:26, <julien.grall@xxxxxxx> wrote:
> > At the moment, the implementation of Set/Way operations will go through
> > all the entries of the guest P2M and flush them. However, this is very
> > expensive and may render unusable a guest OS using them.
> >
> > For instance, Linux 32-bit will use Set/Way operations during secondary
> > CPU bring-up. As the implementation is really expensive, it may be possible
> > to hit the CPU bring-up timeout.
> >
> > To limit the Set/Way impact, we track what pages has been of the guest
> > has been accessed between batch of Set/Way operations. This is done
> > using bit[0] (aka valid bit) of the P2M entry.
> >
> > This patch adds a new per-arch helper is introduced to perform actions just
> > before the guest is first unpaused. This will be used to invalidate the
> > P2M to track access from the start of the guest.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> >
> > ---
> >
> > While we can spread d->creation_finished all over the code, the per-arch
> > helper to perform actions just before the guest is first unpaused can
> > bring a lot of benefit for both architecture. For instance, on Arm, the
> > flush to the instruction cache could be delayed until the domain is
> > first run. This would improve greatly the performance of creating guest.
> Just the other day we had found a potential use on x86 as well
> (even if I already don't recall anymore what it was), so the
> addition is certainly helpful. It might have been nice to split
> introduction of the interface from what you actually want it to
> do on Arm, but irrespective of that
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> for the non-Arm pieces here.
I am expecting the patch to be merged in the next couple of weeks. But I am 
happy to split it if you need it before.
Cheers,

--
Julien Grall

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