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

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Message-ID: <20180720094906.GP2494@hirez.programming.kicks-ass.net>
Date:   Fri, 20 Jul 2018 11:49:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
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

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.

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