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

[Linus Torvalds]

On Mon, Feb 23, 2015 at 1:21 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
>
> On 02/23/2015 04:17 PM, Maciej W. Rozycki wrote:
>>>
>>> It seems highly unlikely to me that a slow FPU instruction can
>>> retire *after* a subsequent fxsave, which would need to happen
>>> for this to work.
>>
>> I meant something else -- a slow FPU instruction can retire after a
>> task has been switched where the FP context has been left intact,
>> i.e. in the lazy FP context switching case, where only the MMU
>> context and GPRs have been replaced.
>
> I don't think that's true, because changing the MMU context and GPRs
> also includes changing the instruction pointer, and changing over the
> execution to the new task.

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.

                        Linus
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