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Re: [PATCH] MIPS: Machine check exception in kmap_coherent()

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH] MIPS: Machine check exception in kmap_coherent()
From: Kevin Cernekee <cernekee@gmail.com>
Date: Mon, 7 Sep 2009 11:28:05 -0700
Cc: linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
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On Mon, Sep 7, 2009 at 3:26 AM, Ralf Baechle<ralf@linux-mips.org> wrote:
> Too complicated.  The fault is happening because the non-SMTC code is trying
> to be terribly clever and pre-loading the TLB with a new wired entry.  On
> SMTC where multiple processors are sharing a single TLB are more careful
> approach is needed so the code does a TLB probe and carefully and re-uses
> an existing entry, if any.  Which happens to be just what we need.
>
> So how about below - only compile tested - patch?

That is an interesting idea.  However, I am not sure we want the IPI
ISR to overwrite copy_user_highpage()'s TLB entry.  That means that
when the interrupt returns, our coherent mapping will likely point to
a different page.  It will avoid the machine check exception, but it
will potentially cause silent, intermittent data corruption instead.

Taking another cue from the SMTC implementation, though - my v2 patch
adds an extra set of fixmap addresses for the in_interrupt() case,
avoiding the VA conflict entirely.  What do you think?

I tested this scheme on non-SMTC.  I suspect that the same change is
required for MT + MP cores like the 1004K, but probably not MT only
cores like 34K which don't use cacheop IPIs.

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