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



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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