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

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Message-ID: <87muumtjwd.fsf@vitty.brq.redhat.com>
Date:   Fri, 20 Jul 2018 10:02:10 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Rik van Riel <riel@...riel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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

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.

(Sorry if I'm missing something important here or if this was already
discussed. I just became aware of this work)

[...]

-- 
  Vitaly