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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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