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

[Shannon Zhao <shannon.zhao@xxxxxxxxxx>]

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.

Thanks,
-- 
Shannon

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