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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Wei,

On 02/08/16 12:01, Wei Liu wrote:
> On Thu, Jul 28, 2016 at 08:42:05PM +0800, Shannon Zhao wrote:
> > On 2016年07月28日 19:06, Julien Grall wrote:
> > > 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?
> > I agree with Stefano, both are fine.
> Sorry for the late reply.
>
> Are you now unblocked? If not, what is not yet decided or needed
> clarification?
I don't think there was a strict consensus. I think this is something we 
can revisit later if necessary as the guest interface does not tie up to 
a specific physical address (The UEFI firmware should retrieve the 
information from the device tree).
So, Shannon could continue towards solution A. I.e the ACPI blob is 
loaded outside of the guest RAM?
If someone disagree please speak up. But we should unblock Shannon to 
get this series in Xen 4.8.
Regards,

--
Julien Grall

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