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

[Shannon Zhao <zhaoshenglong@xxxxxxxxxx>]

On 2016/7/20 17:32, Wei Liu wrote:
> On Wed, Jul 20, 2016 at 02:52:05PM +0800, Shannon Zhao wrote:
>> > 
>> > 
>> > On 2016/7/19 18:38, Wei Liu wrote:
>>> > > On Fri, Jul 15, 2016 at 05:39:32PM +0800, Shannon Zhao wrote:
>>> > > [...]
>>>>>>> > >>> > > 
>>>>>>> > >>> > > It would be trivial to have another option in xl.cfg to allow 
>>>>>>> > >>> > > MB
>>>>>>> > >>> > > granularity. But I don't think that's a good idea. Asking for 
>>>>>>> > >>> > > more
>>>>>>> > >>> > > memory when you don't really know how much is enough is not 
>>>>>>> > >>> > > very useful.
>>>>>>> > >>> > > If an admin can know how much is needed, surely the library 
>>>>>>> > >>> > > can be
>>>>>>> > >>> > > taught to obtain that knowledge, too.
>>>>>>> > >>> > > 
>>>>>>> > >>> > > We need to decide which model we should go with. And, if we 
>>>>>>> > >>> > > decide to
>>>>>>> > >>> > > diverge, document the difference between x86 and ARM model.
>>>>>>> > >>> > > 
>>>>> > >> > Hi Wei,
>>>>> > >> > 
>>>>> > >> > Do you decide how to add the size of ACPI blob to max_memkb?
>>>>> > >> > 
>>> > > AFAICT ARM and x86 maintainers hold different opinions on how memory
>>> > > should be accounted.
>>> > > 
>>> > > I would like to have a unified memory accounting model. But if we can't
>>> > > have that at the moment, I'm fine with divergence, but please document
>>> > > it somewhere (comment near code snippet, in header, or a file under docs
>>> > > etc). And the amount added to max_memkb needs to be properly calculated,
>>> > > not some magic number, so that we have a chance in the future to
>>> > > confidently change how we do thing.
>> > If it's only allowed to add the size to max_memkb in
>> > libxl__domain_build_info_setdefault(), it only can use a const number
>> > since the tables are not generted and we don;t know the size. But the
>> > const number could be chosen properly since we could know the maximum
>> > ACPI tables size of current ARM approach.
>> > 
>> > But maybe in the future, if we add some new ACPI tables, it should
>> > increase the size as well. So I think this should be documented.
>> > 
>> > As I asked before, is it ok to add the size to max_memkb after
>> > generating the ACPI tables and before loading the tables to guest space?
>> > 
> Yes. I don't think shoehorning everything into setdefault is a good
> idea.
> 
> I think libxl_arm.c:libxl__arch_domain_create would be the right place
> to do it.
> 
> I am thinking about calling xc_domain_setmaxmem there, but not adding a
> number to d_config->b_info.max_memkb. Adding that to ->max_memkb would
> be wrong because the bigger ->max_memkb will be recored and the same
> algorithm will be applied every time you migrate your guest, so the
> max_memkb will grow bigger and bigger.
> 
> Given the different approach taken by ARM and x86, maybe we need to also
> record the size of acpi blobs somewhere in xenstore (also needs to be
> documented) so that subsequent libxl_domain_setmaxmem can extract that
> number again.
> 
> Please wait a bit for Stefano and Julien to comment before you do work.
> 
Stefano, Julien,
Any comments regarding how to add the ACPI size to max_memkb?

Thanks,
-- 
Shannon


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