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Date: Tue, 12 Feb 2013 18:13:26 +0200 From: "Michael S. Tsirkin" <mst@...hat.com> To: Paolo Bonzini <pbonzini@...hat.com> Cc: linux-kernel@...r.kernel.org, Wanlong Gao <gaowanlong@...fujitsu.com>, asias@...hat.com, Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org Subject: Re: [PATCH 1/9] virtio: add functions for piecewise addition of buffers On Tue, Feb 12, 2013 at 04:48:39PM +0100, Paolo Bonzini wrote: > Il 12/02/2013 16:43, Michael S. Tsirkin ha scritto: > > On Tue, Feb 12, 2013 at 04:32:27PM +0100, Paolo Bonzini wrote: > >> Il 12/02/2013 15:56, Michael S. Tsirkin ha scritto: > >>>>> +/** > >>>>> + * virtqueue_start_buf - start building buffer for the other end > >>>>> + * @vq: the struct virtqueue we're talking about. > >>>>> + * @data: the token identifying the buffer. > >>>>> + * @nents: the number of buffers that will be added > >>> This function starts building one buffer, number of buffers > >>> is a bit weird here. > >> > >> Ok. > >> > >>>>> + * @nsg: the number of sg lists that will be added > >>> This means number of calls to add_sg ? Not sure why this matters. > >>> How about we pass in in_num/out_num - that is total # of sg, > >>> same as add_buf? > >> > >> It is used to choose between direct and indirect. > > > > total number of in and out should be enough for this, no? > > Originally, I used nsg/nents because I wanted to use mixed direct and > indirect buffers. nsg/nents let me choose between full direct (nsg == > nents), mixed (num_free >= nsg), full indirect (num_free < nsg). Then I > had to give up because QEMU does not support it, but I still would like > to keep that open in the API. Problem is it does not seem to make sense in the API. > In this series, however, I am still using nsg to choose between direct > and indirect. I would like to use dirtect for small scatterlists, even > if they are surrounded by a request/response headers/footers. Shouldn't we base this on total number of s/g entries? I don't see why does it matter how many calls you use to build up the list. > >>>>> +/** > >>>>> + * virtqueue_add_sg - add sglist to buffer being built > >>>>> + * @_vq: the virtqueue for which the buffer is being built > >>>>> + * @sgl: the description of the buffer(s). > >>>>> + * @nents: the number of items to process in sgl > >>>>> + * @dir: whether the sgl is read or written (DMA_TO_DEVICE/DMA_FROM_DEVICE only) > >>>>> + * > >>>>> + * Note that, unlike virtqueue_add_buf, this function follows chained > >>>>> + * scatterlists, and stops before the @nents-th item if a scatterlist item > >>>>> + * has a marker. > >>>>> + * > >>>>> + * Caller must ensure we don't call this with other virtqueue operations > >>>>> + * at the same time (except where noted). > >>> Hmm so if you want to add in and out, need separate calls? > >>> in_num/out_num would be nicer? > >> > >> If you want to add in and out just use virtqueue_add_buf... > > > > I thought the point of this one is maximum flexibility. > > Maximum flexibility does not include doing everything in one call (the > other way round in fact: you already need to wrap with start/end, hence > doing one or two extra add_sg calls is not important). > > Paolo My point is, we have exactly same number of parameters: in + out instead of nsg + direction, and we get more functionality. -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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