Jakub Kicinski <kuba@xxxxxxxxxx> writes:
> On Wed, 13 May 2020 12:25:27 -0700 Andrii Nakryiko wrote:
>> 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
