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Re: framebuffer corruption due to overlapping stp instructions on arm64
- From: Mikulas Patocka <mpatocka at redhat dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Andrew Pinski <pinskia at gmail dot com>, Catalin Marinas <catalin dot marinas at arm dot com>, Will Deacon <will dot deacon at arm dot com>, linux at armlinux dot org dot uk, thomas dot petazzoni at free-electrons dot com, linux-arm-kernel at lists dot infradead dot org, LKML <linux-kernel at vger dot kernel dot org>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Fri, 3 Aug 2018 09:20:40 -0400 (EDT)
- Subject: Re: framebuffer corruption due to overlapping stp instructions on arm64
- References: <alpine.LRH.2.02.1808021242320.31834@file01.intranet.prod.int.rdu2.redhat.com> <CA+=Sn1mWkjuwVnjw6OWWUM=UcP76bdFa680FebCseewHfx3NpA@mail.gmail.com> <9acdacdb-3bd5-b71a-3003-e48132ee1371@redhat.com>
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