netdev - Re: [PATCH bpf-next 1/6] bpf: implement BPF ring buffer and verifier support for it

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Message-ID: <87h7wixndi.fsf@nanos.tec.linutronix.de>
Date:   Thu, 14 May 2020 22:39:05 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jakub Kicinski <kuba@...nel.org>, Andrii Nakryiko <andriin@...com>,
        linux-arch@...r.kernel.org
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, andrii.nakryiko@...il.com,
        kernel-team@...com, "Paul E . McKenney" <paulmck@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH bpf-next 1/6] bpf: implement BPF ring buffer and verifier support for it

Jakub Kicinski <kuba@...nel.org> 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