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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

Thanks, Roger.

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