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

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Date:	Tue, 12 Feb 2013 17:43:38 +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 04:32:27PM +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?

> >> > +/**
> >> > + * 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...
> 
> Paolo

I thought the point of this one is maximum flexibility.
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