Re: [Xen-devel] [PATCH v3 5/6] ioreq-server: add support for multiple servers

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Mar 18, 2014 at 11:33 AM, Paul Durrant <Paul.Durrant@xxxxxxxxxx> wrote:
>> -----Original Message-----
>> From: Ian Campbell
>> Sent: 17 March 2014 14:56
>> To: Paul Durrant
>> Cc: xen-devel@xxxxxxxxxxxxx
>> Subject: Re: [Xen-devel] [PATCH v3 5/6] ioreq-server: add support for
>> multiple servers
>>
>> On Mon, 2014-03-17 at 14:52 +0000, Paul Durrant wrote:
>> > > -----Original Message-----
>> > > From: Ian Campbell
>>
>> > > It's always struck me as odd to have a Xen<->DM communication channel
>> > > sitting there in guest pfn space (regardless of who the nominal owner
>> > > is).
>> > >
>> > > I don't suppose there is any way to pull these pages out of the guest
>> > > pfn space while still accounting them to the guest. Or if there is it
>> > > would probably be a whole other kettle of fish than this series.
>> > >
>> >
>> > The closest analogy I can think of accounting-wise would be shadow
>> > pages. I'll have a look at how they are handled.
>>
>> I think the big difference is that noone outside Xen needs to be able to
>> refer to a shadow page, whereas the device models need some sort of
>> handle onto the ring to be able to map them etc. Not insurmountable I
>> suppose.
>>
>
> Probably not, but it's looking like it will be a bit of a can of worms. Are 
> you ok with sticking to base+range HVM params for secondary emulators that 
> can potentially be moved on migration for now? I.e. the save image just 
> contains a count. There's still some growth room in the existing area (all 
> pages from FE800000 to FF000000 AFACT) so as long as - as George said - we 
> don't bake the PFN layout in, I don't think we preclude moving the emulator 
> PFNs around in future.

xentrace has to share pages between Xen and dom0; it just exposes an
interface for dom0 to get the mfn and then maps those mfns.  Couldn't
you do something similar?  When you create an ioreq server, Xen could
allocate the pages internally; and then you could use
hvm_get_ioreq_server_info to get the MFNs, and xc_map_foreign_range()
to map them.  Am I missing something?

 -George

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