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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: Daniel Jacobowitz <dan@debian.org>
Date: Wed, 4 Dec 2002 12:32:48 -0500
Cc: Carsten Langgaard <carstenl@mips.com>, linux-mips@linux-mips.org, Jun Sun <jsun@mvista.com>
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On Wed, Dec 04, 2002 at 06:08:22PM +0100, Kevin D. Kissell wrote:
> > > I think that Carsten's patch (or equivalent) should certainly be
> > > applied to the main tree, but I wonder how relevant it is here.
> > > The flushes associated with trampolines don't do indexed
> > > flush operations, do they?
> > 
> > True, but are we sure that it's the trampoline that's the problem here?
> 
> Jun Sun seemed to think it was. To quote his original message
> 
> "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."
> 
> I don't know what his actual observations were that lead to that
> conclusion, but the resemblence to what was reported under LTP
> with the pre-break_cow()-patch kernel intrigues me.

Here's some of the actual observations: if you single-step over the
bc1t instruction, then it comes out as you'd expect; the load in the
delay slot was executed.  Even if you breakpoint in the general
vicinity and then continue.

But if you breakpoint _after_ the instruction, it is evident that the
load did not occur as expected.

> So, I repeat... 
> > > ...I don't have a 4Kc platform at
> > > hand, but I think that Jun Sun *may* have found a better
> > > way to get at the other problem I was referring to, which
> > > we rarely saw on non-superscalar issue CPUs, and which
> > > seems to be masked by an otherwise superfluous flush of
> > > the Icache that was added to the latest versions of break_cow().
> > > If Carsten's patch solves the problem without applying that
> > > other update, I'd want to know that.  If it *doesn't*, I'd be
> > > really interested to know if, by any chance, there is a
> > > corelation between failures of Jun Sun's test and the incidence
> > > of page faults on the CACHE op in protected_icache_invalidate_line().
> > >
> > >             Kevin K.
> 
> 
> 

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

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