From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [virtio-dev] packed ring layout proposal v2 Date: Fri, 10 Feb 2017 12:32:49 +0100 Message-ID: References: <20160915223915.qjlnlvf2w7u37bu3@redhat.com> <240c623b-2d8f-28d9-d349-d01e2c24b93a@redhat.com> <20170208214435-mutt-send-email-mst@kernel.org> <20170209202203-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20170209202203-mutt-send-email-mst@kernel.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: virtio-dev@lists.oasis-open.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org On 09/02/2017 19:24, Michael S. Tsirkin wrote: >> I don't know. Power of 2 ring size is pretty standard, I'd rather avoid >> the complication and the gratuitous difference with 1.0. > > I thought originally there's a reason 1.0 rings had to be powers of two > but now I don't see why. OK, we can make it a feature flag later if we > want to. The reason is that it allows indices to be free running. This is an example of QEMU code that requires that: nheads = vring_avail_idx(&vdev->vq[i]) - vdev->vq[i].last_avail_idx; /* Check it isn't doing strange things with descriptor numbers. */ if (nheads > vdev->vq[i].vring.num) { error_report("VQ %d size 0x%x Guest index 0x%x " "inconsistent with Host index 0x%x: delta 0x%x", i, vdev->vq[i].vring.num, vring_avail_idx(&vdev->vq[i]), vdev->vq[i].last_avail_idx, nheads); return -1; } Paolo