Re: [Xen-devel] [PATCH 2/3] x86/pvh: use max_pdx to calculate the paging memory usage

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.10.17 at 12:44, <roger.pau@xxxxxxxxxx> wrote:
> On Fri, Oct 13, 2017 at 08:59:29AM +0000, Jan Beulich wrote:
>> >>> On 13.10.17 at 10:49, <JBeulich@xxxxxxxx> wrote:
>> >>>> On 29.09.17 at 13:25, <roger.pau@xxxxxxxxxx> wrote:
>> >> nr_pages doesn't take into account holes or MMIO regions, and
>> >> underestimates the amount of memory needed for paging. Be on the safe
>> >> side and use max_pdx instead.
>> >> 
>> >> Note that both cases are just approximations, but using max_pdx yields
>> >> a number of free pages after Dom0 build always greater than the
>> >> minimum reserve (either 1/16 of memory or 128MB, whatever is
>> >> smaller).
>> >> 
>> >> Without this patch on a 16GB box the amount of free memory after
>> >> building Dom0 without specifying any dom0_mem parameter would be
>> >> 122MB, with this patch applied the amount of free memory after Dom0
>> >> build is 144MB, which is greater than the reserved 128MB.
>> > 
>> > For the case of there not being a "dom0_mem=" this may indeed
>> > be acceptable (albeit I notice the gap is larger than before, just
>> > this time in the right direction). For the supposedly much more
>> > common case of there being "dom0_mem=" (and with a positive
>> > value), however, not using nr_pages ...
>> >> @@ -288,7 +289,7 @@ unsigned long __init dom0_compute_nr_pages(
>> >>              break;
>> >>  
>> >>          /* Reserve memory for shadow or HAP. */
>> >> -        avail -= dom0_paging_pages(d, nr_pages);
>> >> +        avail -= paging_pgs;
>> > 
>> > ... here is likely going to result in a huge overestimation.
>> 
>> Which I realize may or may not be a problem - the question is
>> whether and if so how far the clamping done by
>> 
>>         nr_pages = min(nr_pages, avail);
>> 
>> above here would result in a meaningfully different amount of
>> memory Dom0 may get for certain command line option / total
>> amount of memory combinations. I.e. quite a bit more than a
>> single data point would need to be provided to prove this isn't
>> going to be perceived as a regression by anyone.
> 
> What about using something like max_pdx - total_pages + nr_pages, that
> seems like a good compromise that should take into account the MMIO
> holes without overestimating as much as just using max_pdx.

I'm not sure - I see no clear correlation between max_pdx,
total_pages, and possible large MMIO space needs. In fact,
max_pdx doesn't express MMIO space needs at all, you'd
need to use max_page in that case.

Jan


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