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Subject: Re: [PATCH v2] MIPS: kernel: Reserve exception base early to prevent
 corruption
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        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
Cc:     Kamal Dasu <kdasu.kdev@gmail.com>,
        Serge Semin <Sergey.Semin@baikalelectronics.ru>
References: <20210306082910.3472-1-tsbogend@alpha.franken.de>
From:   Florian Fainelli <f.fainelli@gmail.com>
Message-ID: <b11d87e6-6665-7579-6334-f9e280dbf20b@gmail.com>
Date:   Sat, 6 Mar 2021 20:13:53 -0800
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On 3/6/2021 12:29 AM, Thomas Bogendoerfer wrote:
> BMIPS is one of the few platforms that do change the exception base.
> After commit 2dcb39645441 ("memblock: do not start bottom-up allocations
> with kernel_end") we started seeing BMIPS boards fail to boot with the
> built-in FDT being corrupted.
> 
> Before the cited commit, early allocations would be in the [kernel_end,
> RAM_END] range, but after commit they would be within [RAM_START +
> PAGE_SIZE, RAM_END].
> 
> The custom exception base handler that is installed by
> bmips_ebase_setup() done for BMIPS5000 CPUs ends-up trampling on the
> memory region allocated by unflatten_and_copy_device_tree() thus
> corrupting the FDT used by the kernel.
> 
> To fix this, we need to perform an early reservation of the custom
> exception space. So we reserve it already in cpu_probe() for the CPUs
> where this is fixed. For CPU with an ebase config register allocation
> of exception space will be done in trap_init().
> 
> Huge thanks to Serget for analysing and proposing a solution to this
> issue.

I made a typo on Sergey's name in my original version here.

> 
> Fixes: 2dcb39645441 ("memblock: do not start bottom-up allocations with kernel_end")
> Reported-by: Kamal Dasu <kdasu.kdev@gmail.com>
> Debugged-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Tested-by: Florian Fainelli <f.fainelli@gmail.com>

Thanks!
-- 
Florian