Re: [Xen-devel] [for-4.9] Re: HVM guest performance regression

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.05.17 at 16:57, <jgross@xxxxxxxx> wrote:
> On 30/05/17 12:43, Jan Beulich wrote:
>>>>> On 30.05.17 at 12:33, <jgross@xxxxxxxx> wrote:
>>> On 30/05/17 09:24, Jan Beulich wrote:
>>>> But if the OS allocated large pages internally for relevant data
>>>> structures, those obviously won't come from that necessarily 4k-
>>>> mapped tail range.
>>>
>>> Sure? I think the kernel is using 1GB pages if possible for direct
>>> kernel mappings of the physical memory. It doesn't care for the last
>>> page mapping some space not populated.
>> 
>> Are you sure? I would very much hope for Linux to not establish
>> mappings to addresses where no memory (and no MMIO) resides.
>> But I can't tell for sure for recent Linux versions; I do know in the
>> old days they were quite careful there.
> 
> Looking at phys_pud_init() they are happily using 1GB pages until they
> have all memory mapped.

It's the layers higher up which I think make sure to call this with
bit PG_LEVEL_1G set only when covering all RAM (see
split_mem_range()).

Jan


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