Messages in this thread | | | Date | Thu, 7 Apr 2005 10:23:07 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: crash in entry.S restore_all, 2.6.12-rc2, x86, PAGEALLOC |
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On Thu, 7 Apr 2005, Dave Jones wrote: > > On Thu, Apr 07, 2005 at 07:47:41AM -0700, Linus Torvalds wrote: > > > So the sysenter sequence might as well look like > > > > pushl $(__USER_DS) > > pushl %ebp > > sti > > pushfl > > .. > > > > which actually does three protected pushes thanks to the one-instruction > > "interrupt shadow" after an sti. > > Is this guaranteed on every x86 variant (or rather, every one > that has SEP). ?
Well, since we only need two in this case, we don't care, but yes, it's supposed to be guaranteed by anything that calls itself an x86.
In fact, we _do_ depend on it in a few other sequences. Notably
sti ; hlt
depends on the fact that an interrupt will always finish _after_ the hlt, and we'll never halt before the hlt (and then re-execute the hlt after the interrupt), and in
sti ; iret
where we depend on the fact that we don't get recursive interrupt stacks (since we at that point have re-enabled the interrupt that happened).
Of course, if some future x86 decides that the interrupt shadow only matters for special instructions (ie it's not so much a general interrupt shadow as a "instruction combination"), I don't think Linux would care. I really think there are only a very few valid sti-combinations, and I suspect the above two are pretty much it.
(The other "magic" x86 behaviour is loading into the SS register, which creates a one-cycle black hole after it. Linux shouldn't care, and in fact nothing should care about it outside of old 16-bit non-protected-mode programs, so I think that's another one that could be retired eventually)
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