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

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Date:	Tue, 18 Dec 2012 14:43:51 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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

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).

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

>> +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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