Re: [Xen-devel] [PATCH 3/3] memory: restrict XENMEM_remove_from_physmap to translated guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.04.19 at 12:26, <julien.grall@xxxxxxx> wrote:
> On 05/03/2019 13:28, Jan Beulich wrote:
>> The commit re-introducing it (14eb3b41d0 ["xen: reinstate previously
>> unused XENMEM_remove_from_physmap hypercall"]) as well as the one having
>> originally introduced it (d818f3cb7c ["hvm: Use main memory for video
>> memory"]) and the one then purging it again (78c3097e4f ["Remove unused
>> XENMEM_remove_from_physmap"]) make clear that this operation is intended
>> for use on HVM (i.e. translated) guests only. Restrict it at least as
>> much, because for PV guests documentation (in the public header) does
>> not even match the implementation: It talks about GPFN as input, but
>> get_page_from_gfn() assumes a GMFN in the non-translated case (and hands
>> back the value passed in).
>> 
>> Also lift the check in XENMEM_add_to_physmap{,_batch} handling up
>> directly into top level hypercall handling, and clarify things in the
>> public header accordingly.
>> 
>> Take the liberty and also replace a pointless use of "current" with a
>> more efficient use of an existing local variable (or function parameter
>> to be precise).
>> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> TBD: It could be further restricted, disallowing its use by a HVM guest
>>       on itself.
> 
> By HVM guest, do you refer to any auto-translated guest?

Yes - sorry for using an x86 term.

> The interface XENME_remove_from_physmap is used by Arm to remove foreign 
> mappings from its p2m. There are potentially other space with similar case 
> (e.g grant-table...).

Oh, I see - this option goes away then.

>> TBD: Is using P2M_ALLOC here really appropriate? It means e.g.
>>       pointlessly populating a PoD slot just to unpopulate it again right
>>       away, with the page then free floating, i.e. no longer available
>>       for use to replace another PoD slot, and (afaict) no longer
>>       accessible by the guest in any way.
>> TBD: Is using guest_physmap_remove_page() here really appropriate? It
>>       means that e.g. MMIO pages wouldn't be removed. Going through
>>       guest_remove_page() (while skipping the de-allocation step) would
>>       seem more appropriate to me, which would address the P2M_ALLOC
>>       aspect above as well.
> 
> How is that an issue? Does XENMEM_add_to_physmap allows you to map MMIO 
> pages?

Well, there's XENMAPSPACE_dev_mmio which xatp handles. But
perhaps the MMIO example is more confusing than helpful. The
question really just is whether guest_remove_page() shouldn't
be used here instead of guest_physmap_remove_page().

But of course - first of all I'd like to get acks (or feedback what to
change) on the actual patch here. The further points would all, if
anything, result in independent patches.

Jan



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