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

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Message-ID: <alpine.LFD.2.11.1502240035310.17311@eddie.linux-mips.org>
Date:	Tue, 24 Feb 2015 00:56:50 +0000 (GMT)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Rik van Riel <riel@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

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.

  Maciej
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