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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 21 Mar 2019 11:38:25 -0700
Message-ID: <CALCETrVUUbSv_pQyVXUSmKV5Jw9DN1y2ib4zGix5Y-9DXw2nGg@mail.gmail.com>
Subject: Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on error_entry
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Jan Beulich <jbeulich@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Joel Fernandes <joel@joelfernandes.org>,
        He Zhe <zhe.he@windriver.com>
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On Thu, Mar 21, 2019 at 11:33 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Mar 21, 2019 at 11:21 AM Andy Lutomirski <luto@kernel.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".
>

I suspect we'll want PV spinlocks and other goodies like PV TLB
shootdown for a long time.  The stuff I'd like to kill eventually is
the PV "I'm not actually at CPL 0 so I'm faking it" part.