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

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Message-ID: <e24c6cae-7b3d-08ce-d15a-87cae90b5f79@suse.com>
Date:   Fri, 11 Aug 2017 15:07:29 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        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>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>, boris.ostrovsky@...cle.com,
        xen-devel@...ts.xenproject.org
Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB
 flush

On 11/08/17 14:54, Peter Zijlstra wrote:
> On Fri, Aug 11, 2017 at 02:46:41PM +0200, Juergen Gross wrote:
>> Aah, okay. Now I understand the problem. The TLB isn't the issue but the
>> IPI is serving two purposes here: TLB flushing (which is allowed to
>> happen at any time) and serialization regarding access to critical pages
>> (which seems to be broken in the Xen case as you suggest).
> 
> Indeed, and now hyper-v as well.

Is it possible to distinguish between non-critical calls of
flush_tlb_others() (which should be the majority IMHO) and critical ones
regarding above problem? I guess the only problem is the case when a
page table can be freed because its last valid entry is gone, right?

We might want to add a serialization flag to indicate flushing _and_
serialization via IPI should be performed.


Juergen