On Mon, Feb 23, 2015 at 4:56 PM, Maciej W. Rozycki <ma...@linux-mips.org> wrote: > 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.
And the save is indeed executed eagerly. Conceivably we could get rid of that on UP, but that seems to be a lot of complexity for extremely little gain. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
