From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark McLoughlin Subject: Re: [PATCH 2/3] virtio: indirect ring entries (VIRTIO_RING_F_INDIRECT_DESC) Date: Tue, 21 Apr 2009 13:59:05 +0100 Message-ID: <1240318745.443.42.camel@blaa> References: <1229620222-22216-1-git-send-email-markmc@redhat.com> <1229620222-22216-2-git-send-email-markmc@redhat.com> <1229620222-22216-3-git-send-email-markmc@redhat.com> Reply-To: Mark McLoughlin Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, Avi Kivity , Dor Laor To: Rusty Russell Return-path: Received: from mx2.redhat.com ([66.187.237.31]:50956 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750874AbZDUNA0 (ORCPT ); Tue, 21 Apr 2009 09:00:26 -0400 In-Reply-To: <1229620222-22216-3-git-send-email-markmc@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: Hi Rusty, On Thu, 2008-12-18 at 17:10 +0000, Mark McLoughlin wrote: > Add a new feature flag for indirect ring entries. These are ring > entries which point to a table of buffer descriptors. > > The idea here is to increase the ring capacity by allowing a larger > effective ring size whereby the ring size dictates the number of > requests that may be outstanding, rather than the size of those > requests. > > This should be most effective in the case of block I/O where we can > potentially benefit by concurrently dispatching a large number of > large requests. Even in the simple case of single segment block > requests, this results in a threefold increase in ring capacity. Apparently, this would also be useful for the windows virtio-net drivers. Dor can explain further, but apparently Windows has been observed passing the driver a packet with >256 fragments when using TSO. With a ring size of 256, the guest can either drop the packet or copy it into a single buffer. We'd much rather if we could use an indirect ring entry to pass this number of fragments without copying. For reference the original patch was here: http://lkml.org/lkml/2008/12/18/212 Cheers, Mark.