On 06/04/18 12:13, George Dunlap wrote:
> On Thu, Apr 5, 2018 at 6:11 PM, Juergen Gross <jgr...@suse.com> wrote:
>>> Option 1: Put the RSDP in lowmem unless we know the guest will use the
>>> address in start_info
>>> Pro: Existing Linux instances boot
>>> Con: Existing BSD instances whose memory is an exact multiple of 1 GiB
>>> will have slightly slower TLB miss times.
>>
>> ... whose memory is >=1GiB ...
> 
> No, the performance benefit only applies to sections of ram that are
> complete 1GiB-aligned sections.  If you have 1.75GiB, then the first
> gigabyte will be a 1GiB page in the p2m (and get the performance
> benefit) but the other 0.75GiB will be 2MiB aligned (and not).

Right. Moving the RSDP out of the first GiB will make it possible to
map that area with a single 1 GiB page.

> We can be pedantic and nail down exactly who will get what benefit
> when, but all it will do is demonstrate how limited this performance
> benefit is.

Moving the shared_info page of HVM guests allowing large pages to be
used increased HVM performance by several percent (e.g. munmap of
user pages was up to 2 times faster this way).


Juergen

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