Re: [PATCH 1/9] virtio: add functions for piecewise addition of buffers

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org,
	Wanlong Gao <gaowanlong@cn.fujitsu.com>,
	asias@redhat.com, Rusty Russell <rusty@rustcorp.com.au>,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 1/9] virtio: add functions for piecewise addition of buffers
Date: Tue, 12 Feb 2013 18:13:26 +0200	[thread overview]
Message-ID: <20130212161326.GB4373@redhat.com> (raw)
In-Reply-To: <511A6457.80609@redhat.com>

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