Jakub Kicinski [off-list ref] writes:

On Wed, 13 May 2020 12:25:27 -0700 Andrii Nakryiko wrote:

quoted

One interesting implementation bit, that significantly simplifies (and thus
speeds up as well) implementation of both producers and consumers is how data
area is mapped twice contiguously back-to-back in the virtual memory. This
allows to not take any special measures for samples that have to wrap around
at the end of the circular buffer data area, because the next page after the
last data page would be first data page again, and thus the sample will still
appear completely contiguous in virtual memory. See comment and a simple ASCII
diagram showing this visually in bpf_ringbuf_area_alloc().

Out of curiosity - is this 100% okay to do in the kernel and user space
these days? Is this bit part of the uAPI in case we need to back out of
it? 

In the olden days virtually mapped/tagged caches could get confused
seeing the same physical memory have two active virtual mappings, or 
at least that's what I've been told in school :)

Yes, caching the same thing twice causes coherency problems.

VIVT can be found in ARMv5, MIPS, NDS32 and Unicore32.

Checking with Paul - he says that could have been the case for Itanium
and PA-RISC CPUs.

Itanium: PIPT L1/L2.
PA-RISC: VIPT L1 and PIPT L2

Thanks,

        tglx