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Re: [Xen-devel] [PATCH 3/3] memory: restrict XENMEM_remove_from_physmap to translated guests
- To: Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Julien Grall <julien.grall@xxxxxxx>
- Date: Tue, 2 Apr 2019 11:26:55 +0100
- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>
- Delivery-date: Tue, 02 Apr 2019 10:27:06 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Hi Jan,
Sorry I was meant to answer to this earlier on.
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?
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...).
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?
Cheers,
--
Julien Grall
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