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

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Date:   Fri, 11 Aug 2017 14:46:41 +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 14:35, Peter Zijlstra wrote:
> On Fri, Aug 11, 2017 at 02:22:25PM +0200, Juergen Gross wrote:
>> Wait - the TLB can be cleared at any time, as Andrew was pointing out.
>> No cpu can rely on an address being accessible just because IF is being
>> cleared. All that matters is the existing and valid page table entry.
>>
>> So clearing IF on a cpu isn't meant to secure the TLB from being
>> cleared, but just to avoid interrupts (as the name of the flag is
>> suggesting).
> 
> Yes, but by holding off the TLB invalidate IPI, we hold off the freeing
> of the concurrently unhooked page-table.
> 
>> In the Xen case the hypervisor does the following:
>>
>> - it checks whether any of the vcpus specified in the cpumask of the
>>   flush request is running on any physical cpu
>> - if any running vcpu is found an IPI will be sent to the physical cpu
>>   and the hypervisor will do the TLB flush there
> 
> And this will preempt a vcpu which could have IF cleared, right?
> 
>> - any vcpu addressed by the flush and not running will be flagged to
>>   flush its TLB when being scheduled the next time
>>
>> This ensures no TLB entry to be flushed can be used after return of
>> xen_flush_tlb_others().
> 
> But that is not a sufficient guarantee. We need the IF to hold off the
> TLB invalidate and thereby hold off the freeing of our page-table pages.

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).

Juergen

>