Date: Fri, 11 Aug 2017 18:26:05 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jork Loeser <Jork.Loeser@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>, Simon Xiao <sixiao@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "luto@kernel.org" <luto@kernel.org>, "hpa@zytor.com" <hpa@zytor.com>,
        "vkuznets@redhat.com" <vkuznets@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "rostedt@goodmis.org" <rostedt@goodmis.org>,
        "andy.shevchenko@gmail.com" <andy.shevchenko@gmail.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@kernel.org" <mingo@kernel.org>,
        "linux-tip-commits@vger.kernel.org" 
        <linux-tip-commits@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB
 flush
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On Fri, Aug 11, 2017 at 09:16:29AM -0700, Linus Torvalds wrote:
> On Fri, Aug 11, 2017 at 2:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > I'm sure we talked about using HAVE_RCU_TABLE_FREE for x86 (and yes that
> > would make it work again), but this was some years ago and I cannot
> > readily find those emails.
> 
> I think the only time we really talked about HAVE_RCU_TABLE_FREE for
> x86 (at least that I was cc'd on) was not because of RCU freeing, but
> because we just wanted to use the generic page table lookup code on
> x86 *despite* not using RCU freeing.
> 
> And we just ended up renaming HAVE_GENERIC_RCU_GUP as HAVE_GENERIC_GUP.
> 
> There was only passing mention of maybe making x86 use RCU, but the
> discussion was really about why the IF flag meant that x86 didn't need
> to, iirc.
> 
> I don't recall us ever discussing *really* making x86 use RCU.

Google finds me this:

  https://lwn.net/Articles/500188/

Which includes:

  http://www.mail-archive.com/kvm@vger.kernel.org/msg72918.html

which does as was suggested here, selects HAVE_RCU_TABLE_FREE for
PARAVIRT_TLB_FLUSH.

But yes, this is very much virt specific nonsense, native would never
need this.