Re: [PATCH] x86/mm: Enable 5-level paging support by default

[Dave Hansen]

On 9/13/19 2:54 AM, Kirill A. Shutemov wrote:
> The next major release of distributions expected to have
> CONFIG_X86_5LEVEL=y.

It's probably worth noting that this exposes to two kinds of possible
performance issues:

First is the overhead of having the 5-level code on 4-level hardware.
We haven't seen any regressions there in quite a while.  Kirill talked
about this in the changelog.

Second is the overhead of having 5-level paging active on 5-level
hardware versus using 4-level paging on hardware *capable* of 5-level.
That is, of course, much harder to measure since 5-level hardware is not
publicly available.  But, we've tested this quite a bit and we're pretty
confident that it will not cause regressions, especially on systems
where apps don't opt in to the larger address space.

I do think endeavoring to have mainline's defaults match the most common
distro configs is a good idea, and now is as good of a time as any.

Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>