linux-kernel - Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on error

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Date:   Thu, 21 Mar 2019 19:27:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Joel Fernandes <joel@...lfernandes.org>,
        He Zhe <zhe.he@...driver.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on
 error_entry

On Thu, Mar 21, 2019 at 11:05:06AM -0700, Andy Lutomirski wrote:

> Ugh.
> 
> I certainly agree in principle that sticking the CR2 read into the asm
> is the right solution.  But this patch makes the spaghetti even more
> tangled.  Maybe we can rearrange the code a bit so that the entry
> sequence saves at least one register before calling error_entry, so we
> can do it the obvious way.

What is the obvious way? Note that with all the Xen/paravirt-me-harder
bits on, we need to clobber at least RAX to get CR2.

And most of the patch was munging the paravirt bits to make that happen.

Another thing I did notices is that CLAC and CLD are not next to one
another in these paths.