linux-kernel - Re: [PATCH RFC 00/15] KVM: Dirty ring interface

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Message-ID: <20191202204344.GB31681@xz-x1>
Date:   Mon, 2 Dec 2019 15:43:44 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 00/15] KVM: Dirty ring interface

On Mon, Dec 02, 2019 at 12:21:19PM -0800, Sean Christopherson wrote:
> On Sat, Nov 30, 2019 at 09:29:42AM +0100, Paolo Bonzini wrote:
> > Hi Peter,
> > 
> > thanks for the RFC!  Just a couple comments before I look at the series
> > (for which I don't expect many surprises).
> > 
> > On 29/11/19 22:34, Peter Xu wrote:
> > > I marked this series as RFC because I'm at least uncertain on this
> > > change of vcpu_enter_guest():
> > > 
> > >         if (kvm_check_request(KVM_REQ_DIRTY_RING_FULL, vcpu)) {
> > >                 vcpu->run->exit_reason = KVM_EXIT_DIRTY_RING_FULL;
> > >                 /*
> > >                         * If this is requested, it means that we've
> > >                         * marked the dirty bit in the dirty ring BUT
> > >                         * we've not written the date.  Do it now.
> > >                         */
> > >                 r = kvm_emulate_instruction(vcpu, 0);
> > >                 r = r >= 0 ? 0 : r;
> > >                 goto out;
> > >         }
> > 
> > This is not needed, it will just be a false negative (dirty page that
> > actually isn't dirty).  The dirty bit will be cleared when userspace
> > resets the ring buffer; then the instruction will be executed again and
> > mark the page dirty again.  Since ring full is not a common condition,
> > it's not a big deal.
> 
> Side topic, KVM_REQ_DIRTY_RING_FULL is misnamed, it's set when a ring goes
> above its soft limit, not when the ring is actually full.  It took quite a
> bit of digging to figure out whether or not PML was broken...

Yeah it's indeed a bit confusing.

Do you like KVM_REQ_DIRTY_RING_COLLECT?  Pair with
KVM_EXIT_DIRTY_RING_COLLECT.  Or, suggestions?

-- 
Peter Xu