Re: [Xen-devel] [V15 PATCH 1/2] pvh dom0: Add and remove foreign pages

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.05.14 at 01:50, <mukesh.rathor@xxxxxxxxxx> wrote:
> On Sat, 24 May 2014 01:08:49 +0200
> Tim Deegan <tim@xxxxxxx> wrote:
> 
>> At 15:37 -0700 on 23 May (1400855820), Mukesh Rathor wrote:
>> > On Fri, 23 May 2014 21:05:34 +0200
>> > Tim Deegan <tim@xxxxxxx> wrote:
>> > 
>> > > At 16:30 -0700 on 22 May (1400772630), Mukesh Rathor wrote:
>> > > > In this patch, a new function, p2m_add_foreign(), is added
>> > > > to map pages from a foreign guest into dom0 for various purposes
>> > > > like domU creation, running xentrace, etc... Such pages are
>> > > > typed p2m_map_foreign.  Note, it is the nature of such pages
>> > > > that a refcnt is held during their stay in the p2m. The
>> > > > refcnt is added and released in the low level ept function
>> > > > atomic_write_ept_entry. That macro is converted to a function to
>> > > > allow for such refcounting, which only applies to leaf entries
>> > > > in the ept. Furthermore, please note that paging/sharing is
>> > > > disabled if the controlling or hardware domain is pvh. Any
>> > > > enabling of those features would need to ensure refcnt are
>> > > > properly maintained for foreign types, or paging/sharing is
>> > > > skipped for foreign types.
>> > > > 
>> > > > Also, we change get_pg_owner so it allows foreign mappings for
>> > > > pvh.
>> > > 
>> > > But you no longer actually call get_pg_owner() for PVH domains,
>> > > right? So that hunk should go away.  With that done,
>> > 
>> > Hi Tim,
>> > 
>> > We actually need get_pg_owner for the mmuext call by the toolstack 
>> > when building a PV domain, doing pinning operations on the guest
>> > table.
>> 
>> Ah, I see.  Let's handle that in a separate patch then, since it's
>> now unrelated to foreign mappings in PVH any more.
>> 
>> Having the change where it is seems fine, but I think the correct test
>> is (is_pv() && paging_mode_translate()) rather than (!is_pvh() &&
>> paging_mode_translate()) -- it's a weakness of the PV pagetable ops
>> that's being avoided here, rather than any special treatment for PVH.
> 
> Good point, but Jan had a concern on that when I had dropped the if
> statement completely, that it would allow HVM guests to go thru. 
> Hence !is_pvh to let hvm guest continue to fail.

The same would be achieved by using is_pv as Tim suggested.

Jan


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