Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

From: Maciej W. Rozycki
Date: Mon Feb 23 2015 - 20:11:14 EST

On Mon, 23 Feb 2015, Linus Torvalds wrote:

> We have one traditional special case, which actually did something
> like Maciej's nightmare scenario: the completely broken "FPU errors
> over irq13" IBM PC/AT FPU linkage.
> But since we don't actually support old i386 machines any more, we
> don't really need to care. The only way you can get into that
> situation is with an external i387. I don't think we need to worry
> about it.
> But with the old horrid irq13 error handling, you literally could get
> into a situation that you got an error "exception" (irq) from the
> previous state, *after* you had already switched to the new one. We
> had some code to mitigate the problem, but as mentioned, I don't think
> it's an issue any more.

Correct, the horrid hack is gone, it was so horrible (though I understand
why IBM had to do it with their PC/AT) that back in mid 1990s, some 10
years after the inception of the problem, Intel felt so compelled to make
people get the handling of it right as to release a dedicated application
note: "AP-578 Software and Hardware Considerations for FPU Exception
Handlers for Intel Architecture Processors", Order Number 243291-002.

Anyway, my point through this consideration has been about the
performance benefit from continuing the execution of an x87 instruction in
parallel, perhaps until after a context has been fully switched. Which
benefit is lost if a FSAVE/FXSAVE executed eagerly at the context switch
stalls waiting for the instruction to complete instead.

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