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Re: possible Malta 4Kc cache problem ...

To: "Kevin D. Kissell" <kevink@mips.com>
Subject: Re: possible Malta 4Kc cache problem ...
From: Carsten Langgaard <carstenl@mips.com>
Date: Wed, 04 Dec 2002 11:08:20 +0100
Cc: linux-mips@linux-mips.org, Jun Sun <jsun@mvista.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20021203224504.B13437@mvista.com> <007501c29b78$f34680e0$10eca8c0@grendel>
Sender: linux-mips-bounce@linux-mips.org
I have just tried your test on a 4Kc and I see no problems.
However I'm running on our internal kernel sources, and as Kevin mention we have
changed a fixed a few things in this area.
As Kevin also mention it sure look more like a I-cache invalidation problem,
rather than a D-cache flush problem, as the 4Kc has a write-through cache.
One think you could try, is our latest kernel release. You can find it here:
ftp://ftp.mips.com/pub/linux/mips/kernel/2.4/images/


/Carsten


"Kevin D. Kissell" wrote:

> > I attached the test case.  Untar it.  Type 'make' and run 'a.out'.
> >
> > If the test fails you will see a print-out.  Otherwise you see nothing.
> >
> > It does not always fail.  But if it fails, it is usually pretty consistent.
> > Try a few times.  Moving source tree to a different directory may cause
> > the symptom appear or disappear.
> >
> > I spent quite some time to trace this problem, and came to suspect
> > there might be a hardware problem.
> >
> > The problem involves emulating a "lw" instruction in cp1 branch delay
> > slot, which needs to  set up trampoline in user stack.  The net effect
> > looks as if the icache line or dcache line is not flushed properly.
> >
> > Using gdb/kgdb, printf or printk in any useful places would hide the bug.
> >
> > I did find a smaller part of the problem.  flush_cache_sigtramp for
> > MIPS32 (4Kc) calls protected_writeback_dcache_line in mips32_cache.h.
> > It uses Hit_Writeback_D, and the 4Kc mannual says it is not implemented
> > and executed as no-op (*ick*).
>
> Which version of the 4Kc manual are you looking at?  I'm looking
> at a very recent version of the 4Kc Software User's Manual
> (version 1.17, dated September 25, 2002), and it only shows
> Hit_Writeback_D to be invalid for *secondary and teritary*
> caches, which makes sense, since the 4KSc doesn't have any.
>
> > Even after fixing this, I still see the problem happening.
>
> That's not too surprising.  The 4Kc D-cache is write-through,
> so if you're really seeing a problem with trampolimes, it is almost
> certain to be a problem with the Icache invalidation, not the
> Dcache flush.
>
> > If you replace flush_cache_sigtramp() with flush_cache_all(), symptom
> > would disppear.
>
> Which again would make sense if there's a problem on
> the icache side of the flush.  Oddly enough, we've seen
> some glitches on other CPUs with other kernels that
> might have been explicable by failures of protected_flush_icache_line(),
> but we never found a problem with it, and a higher-level
> memory management patch made the problem go away.
> Makes me wonder if we shouldn't look at it again, more
> closely.  Is there any possibility that the logic for restarting
> a protected kernel access following a page fault will somehow
> screw up on CACHE instructions, as opposed to the loads
> and stores for which the code was originally written?
>
> > Several of my tests seem to suggest it is the icache that did not
> > get flushed (or updated) properly.
> >
> > Not re-producible on other MIPS boards.  At least so far.
> >
> > Does anybody with more knowledge about 4Kc have any clues here?
> >
> > Thanks.
> >
> > Jun

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