Re: x86: should clear_user() have alternatives?

[Hugh Dickins]

On Tue, 8 Feb 2022, Borislav Petkov wrote:
> Hi Hugh,
> 
> On Mon, Feb 07, 2022 at 09:45:36PM -0800, Hugh Dickins wrote:
> > I realize that dd'ing from /dev/zero to /dev/null, and sparse files on
> > tmpfs, are not prime candidates for optimization; and I've no idea how
> > much clear_user() normally gets used for long clears.
> 
> Right, we usually don't take such "optimizations" because the folks who
> send them always come up with either microbenchmarks or only test on a
> single machine.
> 
> > If I were capable of compiler asm, with alternatives, and knew at what
> > length ERMS becomes advantageous when clearing, I would be sending you
> > a proper patch.  As it is, I'm hoping to tempt someone else to do the
> > work!  Or reject it as too niche to bother with.
> 
> Yap, looking at arch/x86/lib/clear_page_64.S - that's straight-forward
> asm without special-cases noodles like __copy_user_nocache, for example,
> so I wouldn't be opposed to someone
> 
>  - remodelling it so that you can have clear_user* variants there too,
>  with the length supplied so that you can call a common function with
>  arbitrary length and clear_page* can call it too. And then call them in
>  a clear_user() version just like the clear_page() one which selects the
>  proper target based on CPU feature flags.
> 
>  - testing this on bunch of modern machines with, say, a kernel build or
>  some sensible benchmark so that we at least have some coverage
> 
> If the numbers are worth it - and judging by your quick testing above
> they should be - then I don't mind taking that at all.
> 
> If only someone would have the time and diligence to do it properly...

Thanks a lot for setting out what's needed.  Remodelling so there can be
commonality, that's attractive and intriguing.  But I'm disqualified in
several ways (if we tactfully leave out questions of my competence, I'm
still lacking time, and wide machine pool, and expertise with latter):
so I'm responding mainly to make it clear that I shall not be pursuing
this, but hope somebody can eventually.

Thanks,
Hugh

> 
> :-)
> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette