linux-kernel - Re: framebuffer corruption due to overlapping stp instructions on arm64

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Date:   Mon, 6 Aug 2018 13:29:00 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Siddhesh Poyarekar <siddhesh@...plt.org>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Andrew Pinski <pinskia@...il.com>,
        Richard Earnshaw <Richard.Earnshaw@....com>,
        Ramana Radhakrishnan <ramana.gcc@...glemail.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Tulio Magno Quites Machado Filho <tuliom@...ux.ibm.com>
Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64

On 6 August 2018 at 13:19, Siddhesh Poyarekar <siddhesh@...plt.org> wrote:
> On 08/06/2018 04:01 PM, Mikulas Patocka wrote:
>>
>> I think there are three possible solutions:
>>
>> 1. provide an alternative memcpy implementation that doesn't do unaligned
>> accesses and recompile the graphics software with -mstrict-align
>
>
> Given that there's already a tunable glibc.cpu.cached_memopt for powerpc
> that (as Tulio clarified elsewhere) essentially does the same thing for
> cache-inhibited memory, it wouldn't be too much of an overhead to put in
> another ifunc implementation that gets chosen only when one sets this
> tunable.  In fact, we could reuse the C string routines for this to avoid
> adding yet another assembly implementation to have to support. That way we
> can minimally fix the issue at hand without regressing existing uses.
>
> You can then set the glibc.cpu.cached_memopt tunable in the default
> environment for your board[1] or for applications that need it (e.g.
> whenever DISPLAY is exported or something like that).
>
> The only difference from Power would be that cpu.noncached==0 for Power by
> default whereas for aarch64 it will be the other way around.  It shouldn't
> be too hard to enhance the framework to set platform-specific defaults.
>

Thanks Siddhesh,

But we don't need another memcpy(). We need outbound PCIe windows that
tolerate being mapped as normal non-cacheable memory.

And if this is fundamentally impossible, can someone please try
explaining it again? (apologies for being thick)