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

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Date:   Fri, 3 Aug 2018 08:04:32 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
cc:     "'Ard Biesheuvel'" <ard.biesheuvel@...aro.org>,
        Ramana Radhakrishnan <ramana.gcc@...glemail.com>,
        Florian Weimer <fweimer@...hat.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Andrew Pinski <pinskia@...il.com>,
        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>
Subject: RE: framebuffer corruption due to overlapping stp instructions on
 arm64



On Fri, 3 Aug 2018, David Laight wrote:

> From: Ard Biesheuvel
> > Sent: 03 August 2018 10:30
> ...
> > The discussion about whether memcpy() should rely on unaligned
> > accesses, and whether you should use it on device memory is orthogonal
> > to that, and not the heart of the matter IMO
> 
> Even on x86 using memcpy() on PCIe memory (maybe mmap()ed into userspace)
> isn't a good idea.
> In the kernel memcpy_to/fromio() ought to be a better choice but that
> is just an alternate name for memcpy().
> 
> The problem on x86 is that memcpy() is likely to be implemented as
> 'rep movsb' on modern cpu - relying on the cpu hardware to perform
> cache-line sized transfers (etc).
> Unfortunately on uncached locations it has to revert to byte copies.
> So PCIe transfers (especially reads) are very slow.
> 
> The transfers need to use the largest size register available.
> 
> 	David

On x86, the framebuffer is mapped as write-combining memory type, so "rep 
movsb" could merge the byte writes to larger chunks. I don't have a cpu 
with the ERMS feature - could anyone try it if rep movsb works worse or 
better than explicit writes to the framebuffer?

Mikulas