Re: [Xen-devel] [PATCH v6 08/10] xen/arm: Add relinquish_p2m_mapping to remove reference on every mapped page

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 12/18/2013 01:21 PM, Ian Campbell wrote:
> (the cc line here is a bit odd, why Ian J? But not Tim or Stefano? I've
> added those two) On Tue, 2013-12-17 at 17:02 +0000, Ian Campbell wrote:
> > > For the last item, I think it's a bit stupid to create table if we are
> > > removing/relinquish mapping. But I think it's an improvement for later.
> > > There are lots of improvement to do in this function (eg: flushing).
> > Agreed.
> Actually, there is an efficiency concern here.
>
> If we were to skip non-present first and second levels then we would
> skip vast swathes of the address space very quickly. As it stands we
> actively spend time filling it in just so we can recurse over it.
>
> This effectively turns this back into a loop over the entire gfn space,
> which is what we wanted to avoid.
>
> The use of next_gfn_to_relinquish..max_mapped_gfn mitigates this
> somewhat, but max_mapped_gfn is guest controlled and can trivially be
> made huge by the guest.
>
> How hard would it be to skip missing entries for remove and relinquish
> walks? Should just be roughtly:
>
>    int populate = (op == INSERT || op == ALLOCATE)
>
>    for ( addr = ... ; addr < ... ; addr ... )
>    {
>      [...]
>
>      if ( !first[...].valid )
>      {
>         if ( !populate ) {
>          addr += skip size of a first supermapping,
>           continue;
>         }
>         rc = p2m_create(...)
>         [..error..]
>
>         [same for second etc]
>      }
>    }
>
> "size of a first" needs care vs the += PAGE_SIZE in the for loop, I'd be
> inclined to turn this into a while loop and move the += PAGE_SIZE to the
> end.
Your solution should be fine. I will add a patch in the series.

--
Julien Grall

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