On Ne 13-03-05 13:13:16, Linus Torvalds wrote:
>
>
> On Sun, 13 Mar 2005, Stas Sergeev wrote:
> >
> > Such an optimization will cost three more
> > instructions, one of which is a "taken"
> > jump.
>
> I think Pavel missed the fact that you have to check the VM86 bit in
> eflags before you check SS, since otherwise SS doesn't mean anything
> special at all (ie checking for just the normal SS isn't correct: you
> could have a 16-bit SS that looks normal, but is actually a vm86 segment).
>
> Pavel: for the same reason you have to check the low two bits of CS too,
> since if they are zero, then SS hasn't been saved on the stack at all, so
> comparing it against some normal value is meaningless.
Yes, I missed that one, thanks.
What about flag similar to _TIF_SYSCALL_TRACE (call it
_TIF_THIS_BEAST_USES_V86 or something), and only do the tests in the
slowpath if it is set? As normal applications do not use v86, we could
make this 0 instructions in syscall fast path...
Pavel
--
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