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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-07-02 at 10:37 +0100, Julien Grall wrote:
> (Adding Roger)
> 
> On 02/07/14 10:22, Ian Campbell wrote:
> > On Wed, 2014-07-02 at 10:19 +0100, Julien Grall wrote:
> >> Hi Ian,
> >>
> >> On 02/07/14 10:12, Ian Campbell wrote:
> >>> On Tue, 2014-07-01 at 15:57 +0100, Julien Grall wrote:
> >>>> The function domain_get_maximum_gpfn is returning the maximum gpfn ever
> >>>> mapped in the guest. We can use d->arch.p2m.max_mapped_gfn for this 
> >>>> purpose.
> >>>
> >>> What is using the result of this hypercall?
> >>
> >> The result is at least used by xc_dom_gnttab_hvm_seed, to get a scratch
> >> GFN to initialize grant table.
> >>
> >> IHMO this is buggy on ARM (and x86?), because we could have map
> >> everything up to the end of the address space (currently 40 bits).
> > 
> > I wonder if we could find a way to not need this hypercall at all.
> > 
> > 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????

(adding some more x86 folks)

Ian.


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