linux-kernel - Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers

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Date:	Tue, 18 Dec 2012 15:59:38 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	gaowanlong@...fujitsu.com, hutao@...fujitsu.com,
	linux-scsi@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, rusty@...tcorp.com.au,
	asias@...hat.com, stefanha@...hat.com, nab@...ux-iscsi.org
Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of
 buffers

On Tue, Dec 18, 2012 at 02:43:51PM +0100, Paolo Bonzini wrote:
> Il 18/12/2012 14:36, Michael S. Tsirkin ha scritto:
> > Some comments without arguing about whether the performance
> > benefit is worth it.
> > 
> > On Tue, Dec 18, 2012 at 01:32:48PM +0100, Paolo Bonzini wrote:
> >> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> >> index cf8adb1..39d56c4 100644
> >> --- a/include/linux/virtio.h
> >> +++ b/include/linux/virtio.h
> >> @@ -7,6 +7,7 @@
> >>  #include <linux/spinlock.h>
> >>  #include <linux/device.h>
> >>  #include <linux/mod_devicetable.h>
> >> +#include <linux/dma-direction.h>
> >>  #include <linux/gfp.h>
> >>  
> >>  /**
> >> @@ -40,6 +41,26 @@ int virtqueue_add_buf(struct virtqueue *vq,
> >>  		      void *data,
> >>  		      gfp_t gfp);
> >>  
> >> +struct virtqueue_buf {
> >> +	struct virtqueue *vq;
> >> +	struct vring_desc *indirect, *tail;
> > 
> > This is wrong: virtio.h does not include virito_ring.h,
> > and it shouldn't by design depend on it.
> > 
> >> +	int head;
> >> +};
> >> +
> > 
> > Can't we track state internally to the virtqueue?
> > Exposing it seems to buy us nothing since you can't
> > call add_buf between start and end anyway.
> 
> I wanted to keep the state for these functions separate from the rest.
> I don't think it makes much sense to move it to struct virtqueue unless
> virtqueue_add_buf is converted to use the new API (doesn't make much
> sense, could even be a tad slower).

Why would it be slower?

> On the other hand moving it there would eliminate the dependency on
> virtio_ring.h.  Rusty, what do you think?
> 
> >> +int virtqueue_start_buf(struct virtqueue *_vq,
> >> +			struct virtqueue_buf *buf,
> >> +			void *data,
> >> +			unsigned int count,
> >> +			unsigned int count_sg,
> >> +			gfp_t gfp);
> >> +
> >> +void virtqueue_add_sg(struct virtqueue_buf *buf,
> >> +		      struct scatterlist sgl[],
> >> +		      unsigned int count,
> >> +		      enum dma_data_direction dir);
> >> +
> > 
> > And idea: in practice virtio scsi seems to always call sg_init_one, no?
> > So how about we pass in void* or something and avoid using sg and count?
> > This would make it useful for -net BTW.
> 
> It also passes the scatterlist from the LLD.  It calls sg_init_one for
> the request/response headers.
> 
> Paolo

Try adding a _single variant. You might see unrolling a loop
gives more of a benefit than this whole optimization.

> >> +void virtqueue_end_buf(struct virtqueue_buf *buf);
> >> +
> >>  void virtqueue_kick(struct virtqueue *vq);
> >>  
> >>  bool virtqueue_kick_prepare(struct virtqueue *vq);
> >> -- 
> >> 1.7.1
> >>
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