Re: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

[Ingo Molnar]

* Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Wed, Mar 21, 2018 at 6:32 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> >> And even if you ignore that "maintenance problems down the line" issue
> >> ("we can fix them when they happen") I don't want to see games like
> >> this, because I'm pretty sure it breaks the optimized xsave by tagging
> >> the state as being dirty.
> >
> > That's true - and it would penalize the context switch cost of the affected task
> > for the rest of its lifetime, as I don't think there's much that clears XINUSE
> > other than a FINIT, which is rarely done by user-space.
> >
> >> So no. Don't use vector stuff in the kernel. It's not worth the pain.
> >
> > I agree, but:
> >
> >> The *only* valid use is pretty much crypto, and even there it has had issues.
> >> Benchmarks use big arrays and/or dense working sets etc to "prove" how good the
> >> vector version is, and then you end up in situations where it's used once per
> >> fairly small packet for an interrupt, and it's actually much worse than doing it
> >> by hand.
> >
> > That's mainly because the XSAVE/XRESTOR done by kernel_fpu_begin()/end() is so
> > expensive, so this argument is somewhat circular.
> 
> If we do the deferred restore, then the XSAVE/XRSTOR happens at most
> once per kernel entry, which isn't so bad IMO.  Also, with PTI, kernel
> entries are already so slow that this will be mostly in the noise :(

For performance/scalability work we should just ignore the PTI overhead: it 
doesn't exist on AMD CPUs and Intel has announced Meltdown-fixed CPUs, to be 
released later this year:

   https://www.anandtech.com/show/12533/intel-spectre-meltdown

By the time any kernel changes we are talking about today get to distros and users 
the newest hardware won't have the Meltdown bug.

Thanks,

	Ingo