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Date:   Fri, 20 Jul 2018 11:49:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Rik van Riel <riel@surriel.com>,
        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
Message-ID: <20180720094906.GP2494@hirez.programming.kicks-ass.net>
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>
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On Fri, Jul 20, 2018 at 10:02:10AM +0200, Vitaly Kuznetsov wrote:
> 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.

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.