linux-kernel - Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in kvm_make_all_cpus

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Date:   Tue, 11 Apr 2017 15:45:40 -0400 (EDT)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     James Hogan <james.hogan@...tec.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Christoffer Dall <cdall@...aro.org>,
        Andrew Jones <drjones@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in
 kvm_make_all_cpus_request()



----- Original Message -----
> From: "Radim Krčmář" <rkrcmar@...hat.com>
> To: "James Hogan" <james.hogan@...tec.com>
> Cc: "Paolo Bonzini" <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, "Christoffer Dall"
> <cdall@...aro.org>, "Andrew Jones" <drjones@...hat.com>, "Marc Zyngier" <marc.zyngier@....com>, "Christian
> Borntraeger" <borntraeger@...ibm.com>, "Cornelia Huck" <cornelia.huck@...ibm.com>, "Paul Mackerras"
> <paulus@...abs.org>
> Sent: Wednesday, April 12, 2017 3:31:24 AM
> Subject: Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in kvm_make_all_cpus_request()
> 
> 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).

Yeah, I meant "it's wrong in Radim's patches".  Not hard to fix though.

Thanks for the review!

Paolo

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