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

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Message-ID: <a1564e8d091648bcad9b5ec58ab6cc95@AcuMS.aculab.com>
Date:   Fri, 3 Aug 2018 13:04:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mikulas Patocka' <mpatocka@...hat.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

From: Mikulas Patocka
> Sent: 03 August 2018 13:05
...
> > 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?

I don't think 'write combining' can help reads, and memcpy_to/fromio()
are likely to be used for normal memory mapped io areas.

	David

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