On 12/10/2021 09:19, Will Deacon wrote:

On Mon, Oct 11, 2021 at 04:55:37PM +0100, German Gomez wrote:

...

Hi Leo,

On 06/10/2021 10:51, Leo Yan wrote:

...

On Wed, Oct 06, 2021 at 10:35:20AM +0100, German Gomez wrote:

[...]

...

...

So simply say, I think the head pointer monotonically increasing is the right thing to do in Arm SPE driver.

I will talk to James about how we can proceed on this.

Thanks!

I took this offline with James and, though it looks possible to patch the SPE driver to have a monotonically increasing head pointer in order to simplify the handling in the perf tool, it could be a breaking change for users of the perf_event_open syscall that currently rely on the way it works now.

An alternative way we considered to simplify the patch is to change the logic inside the find_snapshot callback so that it records the entire contents of the aux buffer every time.

What do you think?

What does intel-pt do?

Intel-pt has a wrapped head, which is why it has the intel_pt_find_snapshot() function in perf to try to not save any zeros from the buffer that haven't been written yet. (With a wrapped head pointer it's impossible to tell).

Coresight has a monotonically increasing head pointer so it is possible to tell. Recently, Leo removed the Coresight version of find_snapshot() for this reason.

It would be nice to do the same for SPE because that function has a heuristic and is also slow, but I imagine that not returning wrapped head pointers could break anything that expects them.

James

Will