linux-kernel - Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

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Message-ID: <87fu0etdl7.fsf@vitty.brq.redhat.com>
Date:   Fri, 20 Jul 2018 12:18:28 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Rik van Riel <riel@...riel.com>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Mike Galbraith <efault@....de>,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

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

> On Fri, Jul 20, 2018 at 10:02:10AM +0200, Vitaly Kuznetsov wrote:
>> Andy Lutomirski <luto@...capital.net> writes:
>> 
>> > [I added PeterZ and Vitaly -- can you see any way in which this would
>> > break something obscure?  I don't.]
>> 
>> Thanks for CCing me,
>> 
>> I don't see how this can break things either. At first glance, however,
>> I'm afraid we can add performance penalty to virtualized guests which
>> don't use native_flush_tlb_others() (Hyper-V, KVM): we will be reloading
>> CR3 without a need as we don't look at lazy mode in PV tlb flush
>> functions.
>> 
>> We can either check to switch_mm_irqs_off() that
>> native_flush_tlb_others() is in use or teach PV tlb flush functions to
>> look at lazy mode too.
>
> As Rik noted elsewhere in the thread, kvm_flush_tlb_others() ends up
> calling native_tlb_flush_others() for all running vcpu threads.

Ah, right!

>
> The Hyper-V thing is magical, we can't really do anything about it
> there. Let them worry about it.

Well, we kinda know how this magic works: we just ask the hypervisor to
flush TLB for us (if the particular vCPU is running) :-) Anyway, nothing
stops us from duplicating the logic regarding lazy mode from
native_flush_tlb_others() to hyperv_flush_tlb_others(): if TLB state is
lazy omit TLB flush.

-- 
  Vitaly