Re: Does Flushing the Queue after PG REALLY a Necessity?

Randy.Dunlap
2004-02-24T04:59:03Z

On Tue, 24 Feb 2004 10:36:07 +0800 Coywolf Qi Hunt <coywolf@greatcn.org> wrote:

| Philippe Elie wrote:
|
| > On Mon, 23 Feb 2004 at 18:27 +0000, Coywolf Qi Hunt wrote:
| >
| >
| >>H. Peter Anvin wrote:
| >>
| >>
| >>>Anyone happen to know of any legitimate reason not to reload %cs in
| >>>head.S? I think the following would be a lot cleaner, as well as a
| >>>lot safer (the jump and indirect branch aren't guaranteed to have the
| >>>proper effects, although technically neither should be required due to
| >>>the %cr0 write):
| >
| >
| > jump is sufficent when setting PG and required with cpu where cr0 write
| > does not serialize.
|
| The problem is there's two jumps in the kernel. Intel's manual only asks
| for "Execute a near JMP instruction".
|
| >
| >
| >>Anyone happen to know of any legitimate reason to flush the prefetch
| >>queue after enabling paging?
| >>
| >>I've read the intel manual volume 3 thoroughly. It only says that after
| >>entering protected mode, flushing is required, but never says
| >>specifically about whether to do flushing after enabling paging.
| >>
| >>Furthermore the intel example code enables protected mode and paging at
| >>the same time. So does FreeBSD. There's really no more references to check.
| >>
| >> From the cpu's internal view, flushing for PE is to flush the prefetch
| >>queue as well as re-load the %cs, since the protected mode is just about
| >>to begin. But no reason to flushing for PG, since linux maps the
| >>addresses *identically*.
| >>
| >>If no any reason, please remove the after paging flushing queue code,
| >>two near jump.
| >
| >
| > See IA32 vol 3 7.4 and 18.27.3
| >
| > Anyway this code is known to work on dozen of intel/non intel processor,
| > how can you know if changing this code will not break an obscure clone ?
|
| Right, I also think removing the flush code is risky. Thanks very much,
| chapter 18 is what i was looking for. I recalled in an old intel
| booklet, named like something 386 system guide, says JMP after PG as
| well as PE. But I didn't have that book at hand and didn't find any e-doc.

I guess that's the 80386 System Software Writer's Guide.
Ch. 6: Initialization.
Yes, it does JMP after setting PE and after enabling PG.
Any JMP.

| However, in 18.27.3, "The sequence bounded by the MOV and JMP
| instructions should be identity" implies no JMP is also viable
| practically. But we needn't to be that pedantic.
|
| If no any reason for the two jumps, the code should be fixed to remains
| only ONE near jump.

--
~Randy