[RFC,0/2] Introducing trace buffer mapping by user-space

Message

Vincent Donnefort Feb. 12, 2023, 3:32 p.m. UTC

Hi all,

We (Android folks) have been recently working on bringing tracing to the
pKVM hypervisor (more about pKVM? [1] [2]) reusing as much as possible the
tracefs support already available in the host. More specifically, sharing
the ring_buffer_per_cpu between the kernel and the hypervisor, the later
being the writer while the former is only reading. After presenting this
endeavour at the tracingsummit, end of last year [3], Steven observed this
is a similar problem to another idea he had a while ago: mapping the
tracing ring buffers directly into userspace.

The tracing ring-buffer can be stored or sent to network without any copy
via splice. However the later doesn't allow real time processing of the
traces by userspace without a copy, which can only be achieved by letting
userspace map directly the ring-buffer.

And indeed, in both ideas, we have a ring-buffer, an entity being the
writer, the other being a reader and both share the ring buffer pages while
having different VA spaces. So here's an RFC bringing userspace mapping of
a ring-buffer and if it doesn't cover the pKVM hypervisor it nonetheless
brings building blocks that will be reused later.

Any feedback very much appreciated.

Vincent

[1] https://lwn.net/Articles/836693/
[2] https://www.youtube.com/watch?v=9npebeVFbFw
[3] https://tracingsummit.org/ts/2022/hypervisortracing/