linux-kernel - Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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Message-ID: <20191210164749-mutt-send-email-mst@kernel.org>
Date:   Tue, 10 Dec 2019 16:48:59 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, Peter Xu <peterx@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

On Tue, Dec 10, 2019 at 02:31:54PM +0100, Paolo Bonzini wrote:
> On 10/12/19 14:25, Michael S. Tsirkin wrote:
> >> There is no new infrastructure to track the dirty pages---it's just a
> >> different way to pass them to userspace.
> > Did you guys consider using one of the virtio ring formats?
> > Maybe reusing vhost code?
> 
> There are no used/available entries here, it's unidirectional
> (kernel->user).

Didn't look at the design yet, but flow control (to prevent overflow)
goes the other way, doesn't it?  That's what used is, essentially.

> > If you did and it's not a good fit, this is something good to mention
> > in the commit log.
> > 
> > I also wonder about performance numbers - any data here?
> 
> Yes some numbers would be useful.  Note however that the improvement is
> asymptotical, O(#dirtied pages) vs O(#total pages) so it may differ
> depending on the workload.
> 
> Paolo