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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Also please don't forget to Cc maintainers.

Jan


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