Messages in this thread | | | Date | Sat, 21 Feb 2015 10:31:50 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs |
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* Andy Lutomirski <luto@amacapital.net> wrote:
> We have eager and lazy fpu modes, introduced in: > > 304bceda6a18 x86, fpu: use non-lazy fpu restore for processors supporting xsave > > The result is rather messy. There are two code paths in > almost all of the FPU code, and only one of them (the > eager case) is tested frequently, since most kernel > developers have new enough hardware that we use eagerfpu. > > It seems that, on any remotely recent hardware, eagerfpu > is a win: glibc uses SSE2, so laziness is probably > overoptimistic, and, in any case, manipulating TS is far > slower that saving and restoring the full state. > > To try to shake out any latent issues on old hardware, > this changes the default to eager on all CPUs. If no > performance or functionality problems show up, a > subsequent patch could remove lazy mode entirely.
So it would be nice to test this on at least one reasonably old (but not uncomfortably old - say 5 years old) system, to get a feel for what kind of performance impact it has there.
But yes, this would enable a nice simplification in the end so I'm all for it as long as it doesn't cause unacceptable problems - and the FPU code needs simplification badly, because the current latency of bug discovery is too high IMO.
Thanks,
Ingo
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