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

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Date:   Wed, 4 Dec 2019 14:33:57 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Jason Wang <jasowang@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH RFC 00/15] KVM: Dirty ring interface

On Wed, Dec 04, 2019 at 06:39:48PM +0800, Jason Wang wrote:
> 
> On 2019/11/30 上午5:34, Peter Xu wrote:
> > Branch is here:https://github.com/xzpeter/linux/tree/kvm-dirty-ring
> > 
> > Overview
> > ============
> > 
> > This is a continued work from Lei Cao<lei.cao@...atus.com>  and Paolo
> > on the KVM dirty ring interface.  To make it simple, I'll still start
> > with version 1 as RFC.
> > 
> > The new dirty ring interface is another way to collect dirty pages for
> > the virtual machine, but it is different from the existing dirty
> > logging interface in a few ways, majorly:
> > 
> >    - Data format: The dirty data was in a ring format rather than a
> >      bitmap format, so the size of data to sync for dirty logging does
> >      not depend on the size of guest memory any more, but speed of
> >      dirtying.  Also, the dirty ring is per-vcpu (currently plus
> >      another per-vm ring, so total ring number is N+1), while the dirty
> >      bitmap is per-vm.
> > 
> >    - Data copy: The sync of dirty pages does not need data copy any more,
> >      but instead the ring is shared between the userspace and kernel by
> >      page sharings (mmap() on either the vm fd or vcpu fd)
> > 
> >    - Interface: Instead of using the old KVM_GET_DIRTY_LOG,
> >      KVM_CLEAR_DIRTY_LOG interfaces, the new ring uses a new interface
> >      called KVM_RESET_DIRTY_RINGS when we want to reset the collected
> >      dirty pages to protected mode again (works like
> >      KVM_CLEAR_DIRTY_LOG, but ring based)
> > 
> > And more.
> 
> 
> Looks really interesting, I wonder if we can make this as a library then we
> can reuse it for vhost.

So iiuc this ring will majorly for (1) data exchange between kernel
and user, and (2) shared memory.  I think from that pov yeh it should
work even for vhost.

It shouldn't be hard to refactor the interfaces to avoid kvm elements,
however I'm not sure how to do that best.  Maybe like irqbypass and
put it into virt/lib/ as a standlone module?  Would it worth it?

Paolo, what's your take?

-- 
Peter Xu