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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Rik van Riel <riel@surriel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Mike Galbraith <efault@gmx.de>,
        kernel-team <kernel-team@fb.com>, Ingo Molnar <mingo@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier
References: <20180716190337.26133-1-riel@surriel.com>
        <20180716190337.26133-5-riel@surriel.com>
        <CALCETrUMntRe_LYX5wj9YD4xZ+84QExK+ZNb3yxEBDEKa7nePQ@mail.gmail.com>
        <B6566AA4-2E8A-43BD-A224-0F6D4747B8FB@surriel.com>
        <F93A3335-4830-4E36-9179-D0FA5D640C8A@amacapital.net>
        <B976CC13-D014-433A-83DE-F8DF9AB4F421@surriel.com>
        <CALCETrWDiMhgiR3f8n0jdWcW31EDJ+Waq0wh5sMDutfigANGnA@mail.gmail.com>
Date:   Fri, 20 Jul 2018 10:02:10 +0200
In-Reply-To: <CALCETrWDiMhgiR3f8n0jdWcW31EDJ+Waq0wh5sMDutfigANGnA@mail.gmail.com>
        (Andy Lutomirski's message of "Thu, 19 Jul 2018 09:45:47 -0700")
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Andy Lutomirski <luto@amacapital.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