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 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.

Jan


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