linux-kernel - Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping functions

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Message-ID: <ZCWhUJkD0cgSjwkW@google.com>
Date:   Thu, 30 Mar 2023 15:48:48 +0100
From:   Vincent Donnefort <vdonnefort@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping
 functions

[...]

> > > struct ring_buffer_meta_page_header {
> > > #if __BITS_PER_LONG == 64
> > > 	__u64	entries;
> > > 	__u64	overrun;
> > > #else
> > > 	__u32	entries;
> > > 	__u32	overrun;
> > > #endif
> > > 	__u32	pages_touched;
> > > 	__u32	meta_page_size;
> > > 	__u32	reader_page;	/* page ID for the reader page */
> > > 	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
> > > };
> > > 
> > > BTW, shouldn't the nr_data_pages take into account the reader page? As it
> > > is part of the array we traverse isn't it?  
> > 
> > It depends if the reader page has ever been swapped out. If yes, the reader
> > would have to start from reader_page and then switch to the data_pages.
> > Which sounds like a fiddly interface for the userspace.
> > 
> > So yeah, consuming-read only feels like a better start.
> > 
> 
> I agree. I'd like to get something in that can be extended, but simple
> enough that it's not too much of a barrier wrt getting the API correct.
> 
> -- Steve

Something I just realized though. In the event of being able to upstream the
hypervisor tracing based on the ring_buffer_meta_page, without non-consumming
support, we wouldn't have the "trace" file which is used to reset the buffers.

I'd guess we'd have to either create one that is read-only (a bit strange) or
let trace_pipe reset the buffer(s).