linux-kernel - Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

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Date:   Mon, 14 Aug 2017 15:20:49 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Simon Xiao <sixiao@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "luto\@kernel.org" <luto@...nel.org>,
        "hpa\@zytor.com" <hpa@...or.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt\@goodmis.org" <rostedt@...dmis.org>,
        "andy.shevchenko\@gmail.com" <andy.shevchenko@...il.com>,
        "tglx\@linutronix.de" <tglx@...utronix.de>,
        "mingo\@kernel.org" <mingo@...nel.org>,
        "linux-tip-commits\@vger.kernel.org" 
        <linux-tip-commits@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

Peter Zijlstra <peterz@...radead.org> writes:

> 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@...radead.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.

In case we decide to go HAVE_RCU_TABLE_FREE for all PARAVIRT-enabled
kernels (as it seems to be the easiest/fastest way to fix Xen PV) - what
do you think about the required testing? Any suggestion for a
specifically crafted micro benchmark in addition to standard
ebizzy/kernbench/...?

Additionally, I see another option for us: enable 'rcu table free' on
boot (e.g. by taking tlb_remove_table to pv_ops and doing boot-time
patching for it) so bare metal and other hypervisors are not affected
by the change.

-- 
  Vitaly