RE: framebuffer corruption due to overlapping stp instructions on arm64

Mikulas Patocka
2018-08-05T14:36:11Z

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
like movdqa.

> are likely to be used for normal memory mapped io areas.
>
> David

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.

Mikulas