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Date: Tue, 11 Apr 2017 21:31:24 +0200
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
To: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, Christoffer Dall <cdall@linaro.org>,
        Andrew Jones <drjones@redhat.com>, Marc Zyngier <marc.zyngier@arm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Cornelia Huck <cornelia.huck@de.ibm.com>,
        Paul Mackerras <paulus@ozlabs.org>
Subject: Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in
 kvm_make_all_cpus_request()
Message-ID: <20170411193124.GA395@potion>
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2017-04-11 10:37+0100, James Hogan:
> Hi Paolo,
> 
> On Tue, Apr 11, 2017 at 01:25:04PM +0800, Paolo Bonzini wrote:
>> On 07/04/2017 05:02, James Hogan wrote:
>> > This presumably changes the behaviour on x86, from != OUTSIDE_GUEST_MODE
>> > to == IN_GUEST_MODE. so:
>> > - you'll no longer get IPIs if its in READING_SHADOW_PAGE_TABLES (which
>> >   MIPS also now uses when accessing mappings outside of guest mode and
>> >   depends upon to wait until the old mappings are no longer in use).
>> 
>> This is wrong, the purpose of READING_SHADOW_PAGE_TABLES is "kvm_flush_remote_tlbs
>> should send me an IPI, because I want to stop kvm_flush_remote_tlbs until I'm done
>> reading the page tables".
> 
> That sounds equivalent to what I meant for MIPS, i.e.
> kvm_flush_remote_tlbs() does the waiting (not the thing accessing guest
> mappings).

I agree, thanks for noticing this.  It would be a huge mistake to drop
the synchronization.