Linus Torvalds wrote:
On Sun, 4 Nov 2007, Linus Torvalds wrote:
I'm not entirely sure that it needs to be a long-jump, btw. I think any
regular branch is sufficient. You obviously *do* need to make the long
jump later (to reload %cs in protected mode), but I'm not sure it's needed
in that place. I forget the exact rules (but they definitely were
documented).
Hmm. The original Linux code did
movw $1, %ax
lmsw %ax
jmp flush_instr
flush_instr:
and I think that was straigh out of the documentation. So yeah, I think
that's the right fix - not a longjmp (which in itself is dangerous: it
potentially behaves *differently* on different CPU's, since some CPU's may
do the long jump with pre-protected-mode semantics, while others will do
it with protected mode already in effect!)
Just looked it up; it was a bit hard to find (it is Intel vol 3 page
9-27, at least in the version I have), but you're right -- the
documentation only demands a short jump here, not a long jmp (which
actually makes sense given what I remembered that a long jump should be
deferrable here.) So yes, that is definitely the right fix and avoids
the ugly mixing of code.
I'll update the patch.
-hpa
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