linux-kernel - Re: [PATCH v7 06/14] KVM: Make dirty ring exclusive to dirty bitmap log

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Date:   Mon, 23 Mar 2020 12:16:26 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Christophe de Dinechin <dinechin@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v7 06/14] KVM: Make dirty ring exclusive to dirty bitmap
 log

On Sat, Mar 21, 2020 at 12:12:50PM -0700, Sean Christopherson wrote:
> On Wed, Mar 18, 2020 at 12:37:12PM -0400, Peter Xu wrote:
> > There's no good reason to use both the dirty bitmap logging and the
> > new dirty ring buffer to track dirty bits.  We should be able to even
> > support both of them at the same time, but it could complicate things
> > which could actually help little.  Let's simply make it the rule
> > before we enable dirty ring on any arch, that we don't allow these two
> > interfaces to be used together.
> > 
> > The big world switch would be KVM_CAP_DIRTY_LOG_RING capability
> > enablement.  That's where we'll switch from the default dirty logging
> > way to the dirty ring way.  As long as kvm->dirty_ring_size is setup
> > correctly, we'll once and for all switch to the dirty ring buffer mode
> > for the current virtual machine.
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >  Documentation/virt/kvm/api.rst |  7 +++++++
> >  virt/kvm/kvm_main.c            | 12 ++++++++++++
> >  2 files changed, 19 insertions(+)
> > 
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 99ee9cfc20c4..8f3a83298d3f 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6202,3 +6202,10 @@ make sure all the existing dirty gfns are flushed to the dirty rings.
> >  
> >  The dirty ring can gets full.  When it happens, the KVM_RUN of the
> >  vcpu will return with exit reason KVM_EXIT_DIRTY_LOG_FULL.
> > +
> > +NOTE: the KVM_CAP_DIRTY_LOG_RING capability and the new ioctl
> 
> Leave off "new", it'll be stale a few months/years from now.

Ok.

> 
> > +KVM_RESET_DIRTY_RINGS are exclusive to the existing KVM_GET_DIRTY_LOG
> 
> Did you mean "mutually exclusive with"?  "exclusive to" would mean they
> can only be used by KVM_GET_DIRTY_LOG with doesn't match the next
> sentence.

I meant "mutual-exclusive".  I'll fix it up.

Thanks,

-- 
Peter Xu