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Date:   Fri, 3 Aug 2018 09:20:40 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Florian Weimer <fweimer@redhat.com>
cc:     Andrew Pinski <pinskia@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
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        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>,
        GNU C Library <libc-alpha@sourceware.org>
Subject: Re: framebuffer corruption due to overlapping stp instructions on
 arm64
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On Fri, 3 Aug 2018, Florian Weimer wrote:

> On 08/03/2018 09:11 AM, Andrew Pinski wrote:
> > Yes fix Links not to use memcpy on the framebuffer.
> > It is undefined behavior to use device memory with memcpy.
> 
> Some (de facto) ABIs require that it is supported, though.  For example, 
> the POWER string functions avoid unaligned loads and stores for this 
> reason because the platform has the same issue with device memory.  And 
> yes, GCC will expand memcpy on POWER to something that is incompatible 
> with device memory. 8-(
> 
> If we don't want people to use memcpy, we probably need to provide a 
> credible alternative.
> 
> Thanks,
> Florian

And what does POWER do with code like this?

void write_merge(int *x)
{
        x[0] = x[1] = 0;
}

With -O2, gcc-8 translates it into:

        li 9,0
        std 9,0(3)
        blr

And that std instruction may end up being unaligned (the C ABI mandates 
that x is aligned to 4 bytes, not 8).

If this piece of code is inside some graphics driver and writes to 
framebuffer memory, what do you do with it?

Mikulas