Re: [Xen-devel] [PATCH] xen/arm: Implement domain_get_maximum_gpfn

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-07-02 at 10:50 +0100, Jan Beulich wrote:
> >>> On 02.07.14 at 11:41, <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Wed, 2014-07-02 at 10:37 +0100, Julien Grall wrote:
> >> On 02/07/14 10:22, Ian Campbell wrote:
> >> > Any reason why both arm and x86 can't just use a fixed scratch pfn for
> >> > this temporary mapping? Both of them surely have spaces which they can
> >> > guarantee won't overlap with anything.
> >> 
> >> This was the previous behavior until last November.
> >> 
> >> commit db062c28f30eb68d1b5d7a910445a0ba1136179a
> >> Date:   Wed Nov 13 09:26:13 2013 +0100
> >> 
> >>     libxc: move temporary grant table mapping to end of memory
> >>     
> >>     In order to set up the grant table for HVM guests, libxc needs to map
> >>     the grant table temporarily.  At the moment, it does this by adding the
> >>     grant page to the HVM guest's p2m table in the MMIO hole (at gfn 
> >> 0xFFFFE),
> >>     then mapping that gfn, setting up the table, then unmapping the gfn and
> >>     removing it from the p2m table.
> >>     
> >>     This breaks with PVH guests with 4G or more of ram, because there is
> >>     no MMIO hole; so it ends up clobbering a valid RAM p2m entry, then
> >>     leaving a "hole" when it removes the grant map from the p2m table.
> >>     Since the guest thinks this is normal ram, when it maps it and tries
> >>     to access the page, it crashes.
> >>     
> >>     This patch maps the page at max_gfn+1 instead.
> >> 
> >> I'm not sure what to do for x86, so I was planning to introduce a per-arch 
> > hook to retrieve a scratch gpfn.
> >> x86 would keep the current behavior, and ARM will use the GNTTAB space in 
> > the layout.
> > 
> > Perhaps x86 could use some well known MMIO space, like the APIC at
> > 0xfff????
> 
> Except that PVH has no LAPIC right now. Yet with the recent hole
> punching patches I wonder whether "there is no MMIO hole" is actually
> correct. Roger?

Another option might be to just burn a special pfn on a scratch mapping.

Ian.


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