linux-kernel - Re: [PATCH v3 00/21] KVM: Dirty ring interface

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Message-ID: <feffe012-0407-128a-185a-42cb9d5aac5c@redhat.com>
Date:   Thu, 9 Jan 2020 21:51:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, Peter Xu <peterx@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Christophe de Dinechin <dinechin@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Kevin <kevin.tian@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>
Subject: Re: [PATCH v3 00/21] KVM: Dirty ring interface

On 09/01/20 20:13, Michael S. Tsirkin wrote:
> That's one of the reasons I called for using something
> resembling vring_packed_desc.

In principle it could make sense to use the ring-wrap detection
mechanism from vring_packed_desc instead of the producer/consumer
indices.  However, the element address/length indirection is unnecessary.

Also, unlike virtio, KVM needs to know if there are N free entries (N is
~512) before running a guest.  I'm not sure if that is possible with
ring-wrap counters, while it's trivial with producer/consumer indices.

Paolo