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RE: framebuffer corruption due to overlapping stp instructions on arm64
- From: Mikulas Patocka <mpatocka at redhat dot com>
- To: David Laight <David dot Laight at ACULAB dot COM>
- Cc: "'Ard Biesheuvel'" <ard dot biesheuvel at linaro dot org>, Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>, Florian Weimer <fweimer at redhat dot com>, Thomas Petazzoni <thomas dot petazzoni at free-electrons dot com>, GNU C Library <libc-alpha at sourceware dot org>, 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>, Russell King <linux at armlinux dot org dot uk>, LKML <linux-kernel at vger dot kernel dot org>, linux-arm-kernel <linux-arm-kernel at lists dot infradead dot org>
- Date: Sun, 5 Aug 2018 10:36:01 -0400 (EDT)
- Subject: RE: framebuffer corruption due to overlapping stp instructions on arm64
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On Fri, 3 Aug 2018, David Laight wrote:
> 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()
There's an instruction movntdqa (and vmovntdqa) that can actually do
prefetch on write-combining memory type. It's the only instruction that
can do it.
It this instruction is used on non-write-combining memory type, it behaves
> are likely to be used for normal memory mapped io areas.
I benchmarked it on a processor with ERMS - for writes to the framebuffer,
there's no difference between memcpy, 8-byte writes, rep stosb, rep stosq,
mmx, sse, avx - all this method achieve 16-17 GB/s
For reading from the framebuffer:
323 MB/s - memcpy (using avx2)
91 MB/s - explicit 8-byte reads
249 MB/s - rep movsq
307 MB/s - rep movsb
90 MB/s - mmx
176 MB/s - sse
4750 MB/s - sse movntdqa
330 MB/s - avx
5369 MB/s - avx vmovntdqa
So - it may make sense to introduce a function memcpy_from_framebuffer()
that uses movntdqa or vmovntdqa on CPUs that support it.