Re: [Xen-devel] [PATCH v2 16/17] libxc/xc_dom_arm: Copy ACPI tables to guest space

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 26/07/16 02:17, Boris Ostrovsky wrote:
> On 07/25/2016 07:40 PM, Stefano Stabellini wrote:
> > On Mon, 25 Jul 2016, Boris Ostrovsky wrote:
> > > On 07/25/2016 06:06 PM, Stefano Stabellini wrote:
> > > > On Mon, 25 Jul 2016, George Dunlap wrote:
> > > > > On Thu, Jul 21, 2016 at 10:15 PM, Stefano Stabellini
> > > > > <sstabellini@xxxxxxxxxx> wrote:
> > > > Going back to the discussion about how to account for the ACPI blob in
> > > > maxmem, let's make this simple, if we increase maxmem by the size of
> > > > the
> > > > ACPI blob:
> > > >
> > > > - the toolstack allocates more RAM than expected (bad)
> > > > - when the admin specifies 1GB of RAM, the guest actually gets 1GB of
> > > >    usable RAM (good)
> > > > - things are faster as Xen and the guest can exploit superpage mappings
> > > >    more easily at stage-1 and stage-2 (good)
> > > >
> > > > Let's call this option A.
> > > >
> > > > If we do not increase maxmem:
> > > >
> > > > - the toolstack allocates less RAM, closer to the size specified in the
> > > >    VM config file (good)
> > > > - the guest gets less usable memory than expected, less than what was
> > > >    specified in the VM config file (bad)
> > > Not sure I agree with this, at least for x86/Linux: guest gets 1GB of
> > > usable RAM and part of that RAM stores ACPI stuff. Guest is free to
> > > stash ACPI tables somewhere else or ignore them altogether and use that
> > > memory for whatever it wants.
> > On ARM it will be a ROM (from guest POV)
>
> In which case I don't see why we should take it from maxmem allocation.
> I somehow thought that there was a choice of whether to put it in ROM or
> RAM on ARM but if it's ROM only then I don't think there is an option.
We have option to do the both on ARM. I just feel that the ROM option is 
a cleaner interface because the ACPI tables are not supposed be modified 
by the guest, so we can prevent to be overridden (+ all the advantages 
mentioned by Stefano with option A).
> IIUIC the toolstack pretends that the blob goes to memory because that's
> how its interfaces work but that space is not really what we think about
> when we set memory/maxmem in the configuration file. Unlike x86.
I think we need to draw a conclusion for Shannon to continue to do the 
work and I would like to see this series in Xen 4.8. From my 
understanding you are for option B, so does George.
Stefano votes for option A, but find B acceptable. Any other opinions?

Regards,

--
Julien Grall

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