Re: Ingo's PIII FXSTOR patch [2 Mar] causes problems

Linus Torvalds (torvalds@transmeta.com)
16 Apr 1999 04:24:52 GMT


In article <371693F9.EE78C6FE@redhat.com>,
Doug Ledford <dledford@redhat.com> wrote:
>
>I'm currently furthering Ingo's original work on this (it was the basis
>for some code that was used in the March 17th PIII Xeon launch event by
>Intel). As it turns out, if you really pay attention to detail, it is
>*impossible* to reconstruct the complete FPU state that you would
>normally get from fsave using only fxsave. The changes to the Tag Word
>make it impossible (the fxsave uses a lossy form of saving the tag word
>and we can't get that lost information back).

I haven't looked into the exact details, but I guess you're referring to
the fact that fxsave only saves the "empty" bit.

That's not lossy: the rest of the tag information is implicit in the
actual value of the stack entry.

It shoul dbe really easy to convert between the new and the old format:
let the hardware do it for you:

fxrestore new-format
fnsave old-format

I don't see any loss of information anywhere.. Am I overlooking
something?

Linus

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/