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From: =?ISO-8859-1?Q?Roger_Pau_Monn=E9?= <roger.pau@citrix.com>
Subject: Re: [PATCH] xen/arm: Implement domain_get_maximum_gpfn
Date: Wed, 2 Jul 2014 12:19:43 +0200
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To: Jan Beulich <JBeulich@suse.com>, Ian Campbell <Ian.Campbell@citrix.com>, Julien Grall <julien.grall@linaro.org>
Cc: xen-devel@lists.xenproject.org, Stefano Stabellini <stefano.stabellini@citrix.com>, Tim Deegan <tim@xen.org>
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On 02/07/14 11:50, Jan Beulich wrote:
>>>> On 02.07.14 at 11:41, <Ian.Campbell@citrix.com> 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?

For PVH guests there's still no MMIO hole (or any other kind of hole) at
all, the hole(s) is only there for Dom0.

Roger.