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

[Ingo Molnar]

* Dave Hansen <dave.hansen@xxxxxxxxx> wrote:

> 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>

Ok - in terms of timing it's obviously *way* too late for v5.4, so I've 
queued it up for the v5.5 merge window in tip:x86/mm. This should give it 
2-3 months of additional testing to shake out any weird interactions and 
quirks.

Thanks,

	Ingo