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

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Message-ID: <20190321183713.GX5996@hirez.programming.kicks-ass.net>
Date:   Thu, 21 Mar 2019 19:37:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Jan Beulich <jbeulich@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>
Subject: Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on
 error_entry

On Thu, Mar 21, 2019 at 11:25:44AM -0700, Linus Torvalds wrote:
> On Thu, Mar 21, 2019 at 11:21 AM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > I dunno.  Lots of people at least use to have serious commercial interest in it.
> 
> Yes, it used to be a big deal. But full virtualization has gotten a
> lot more common and better.
> 
> > Hey Xen folks, how close are we to being able to say "if you want to
> > run a new kernel, you need to switch to PVH or similar"?
> 
> I'd also like to know if we could perhaps at least limit PV to just
> the thing that people care most deeply about.
> 
> For example, maybe people notice that they really deeply care about
> the PV spinlocks because they help a lot for some loads, but don't
> care so much about the low-level CPU PV stuff any more because modern
> CPUs do _those_ things so well these days.
> 
> So it might not be an all-or-nothing thing, but a gradual "let's stop
> supporting xyz under PV, because it causes pain and isn't worth it".

So Juergen recently introduced PARAVIRT_XXL, which are exactly those
bits of PV we can get rid of.

This paravirt-me-harder config does indeed include the CR2 bits.

I recently talked to Andrew Cooper about this, and he said Xen Dom0
still needs all this :/

But yes, I'd _love_ to delete all that in a hurry.