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Date:   Sun, 7 Mar 2021 23:27:14 +0100 (CET)
From:   "Maciej W. Rozycki" <macro@orcam.me.uk>
To:     Serge Semin <Sergey.Semin@baikalelectronics.ru>
cc:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Serge Semin <fancer.lancer@gmail.com>,
        Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org, Kamal Dasu <kdasu.kdev@gmail.com>
Subject: Re: [PATCH v2] MIPS: kernel: Reserve exception base early to prevent
 corruption
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On Mon, 8 Mar 2021, Serge Semin wrote:

> > some of them are not R2 (SB1), others are. So best bet would be to
> > simply reserve the first 0x1000 bytes for every CPU and special handling
> > for the BMIPS case. Does this cover all cases ?
> 
> I can't say for sure whether it will cover all the cases/platforms. I
> visually analysed all the
> board_{nmi_handler,ejtag_handler,ebase,cache_error}_setup callbacks
> implementation in MIPS arch to create the list above. Exception vectors or
> some other stuff can be setup in some other platform-specific manner. But at
> least reserving a memory below PAGE_SIZE would get the situation partly back
> to before the memory below the kernel stopped being reserved. Hopefully
> one page will be enough for the platforms, which relied on that rule. The
> rest or them sooner or later will manifest itself as it has happened with
> Broadcom.

 I think reserving up to 64KiB might be a bit excessive on one hand while 
having the size of the reservation depend on configured PAGE_SIZE could be 
too unpredictable on the other.  I think 4KiB is a good compromise and I'd 
leave anything else for platform maintainers to sort out.

  Maciej