linux-kernel - Re: [PATCH 0/8] tracing: Persistent traces across a reboot or crash

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Date: Tue, 19 Mar 2024 20:44:10 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Joel
 Fernandes <joel@...lfernandes.org>, Masami Hiramatsu <mhiramat@...nel.org>,
 Mark Rutland <mark.rutland@....com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Vincent Donnefort <vdonnefort@...gle.com>,
 Daniel Bristot de Oliveira <bristot@...hat.com>, Ingo Molnar
 <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 suleiman@...gle.com, Thomas Gleixner <tglx@...utronix.de>, Vineeth Pillai
 <vineeth@...byteword.org>, Youssef Esmat <youssefesmat@...gle.com>, Beau
 Belgrave <beaub@...ux.microsoft.com>, Alexander Graf <graf@...zon.com>,
 Baoquan He <bhe@...hat.com>, Borislav Petkov <bp@...en8.de>, "Paul E.
 McKenney" <paulmck@...nel.org>, David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 0/8] tracing: Persistent traces across a reboot or crash

On Sat, 9 Mar 2024 12:40:51 -0800
Kees Cook <keescook@...omium.org> wrote:

> The part I'd like to get wired up sanely is having pstore find the
> nvdimm area automatically, but it never quite happened:
> https://lore.kernel.org/lkml/CAGXu5jLtmb3qinZnX3rScUJLUFdf+pRDVPjy=CS4KUtW9tLHtw@mail.gmail.com/

The automatic detection is what I'm looking for.

> 
> > Thanks for the info. We use pstore on ChromeOS, but it is currently
> > restricted to 1MB which is too small for the tracing buffers. From what
> > I understand, it's also in a specific location where there's only 1MB
> > available for contiguous memory.  
> 
> That's the area that is specifically hardware backed with persistent
> RAM.
> 
> > I'm looking at finding a way to get consistent memory outside that
> > range. That's what I'll be doing next week ;-)
> > 
> > But this code was just to see if I could get a single contiguous range
> > of memory mapped to ftrace, and this patch set does exactly that.  
> 
> Well, please take a look at pstore. It should be able to do everything
> you mention already; it just needs a way to define multiple regions if
> you want to use an area outside of the persistent ram area defined by
> Chrome OS's platform driver.

I'm not exactly sure how to use pstore here. At boot up I just need some
consistent memory reserved for the tracing buffer. It just needs to be the
same location at every boot up.

I don't need a front end. If you mean a way to access it from user space.
The front end is the tracefs directory, as I need all the features that the
tracefs directory gives.

I'm going to look to see how pstore is set up in ChromeOS and see if I can
use whatever it does to allocate another location.

-- Steve