Re: [Xen-devel] [RFC 16/16] xen/arm: Track page accessed between batch of Set/Way operations

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 09/10/2018 08:04, Jan Beulich wrote:
> > > > On 08.10.18 at 20:33, <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.
> While I'm not opposed to the new arch hook, why don't you create the
> p2m entries in their intended state right away? At the very least this
> would have the benefit of confining the entire change to Arm code.
Let me start by saying I think having a hook to perform an action once 
the VM has been fully created is quite useful. For instance, this could 
be used on Arm to limit the invalidation of the icache. At the moment, 
we invalidate the icache for every populate memory hypercall. This is 
quite a waste of cycle.
In this particular circumstance, I would still like to use the hardware 
for walking the page-tables during the domain creation (i.e when copy 
binary over). This would not be possible if we create the entry with 
valid bit unset.
Furthermore, we don't need to create entry with valid bit unset once the 
guest is running. So we would need to check in the P2M code whether the 
guest is running and whether IOMMU is enabled.
So overall, I fell this is the best way to keep it simple for Arm and 
open door to speed/streamline a bit more the domain creation.
Cheers,

--
Julien Grall

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