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    SubjectRe: [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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