linux-kernel - Re: Re: [PATCH 1/2] virtio: abstract virtqueue related methods

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Message-ID: <37a5e7dc-160d-51d2-7631-196ad9e21da7@bytedance.com>
Date:   Fri, 12 May 2023 19:09:40 +0800
From:   zhenwei pi <pizhenwei@...edance.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     stefanha@...hat.com, jasowang@...hat.com,
        xuanzhuo@...ux.alibaba.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH 1/2] virtio: abstract virtqueue related methods

On 5/12/23 18:46, Michael S. Tsirkin wrote:
> On Fri, May 12, 2023 at 05:46:17PM +0800, zhenwei pi wrote:
>> There is already a virtqueue abstract structure in virtio subsystem
>> (see struct virtqueue in include/linux/virtio.h), however the vring
>> based virtqueue is used only in the past years, the virtqueue related
>> methods mix much with vring(just look like the codes, virtqueue_xxx
>> functions are implemented in virtio_ring.c).
>>
>> Abstract virtqueue related methods(see struct virtqueue_ops), and
>> separate virtqueue_xxx symbols from vring. This allows a non-vring
>> based transport in the future. With this change, the following symbols
>> are exported from virtio.ko instead of virtio_ring.ko:
>>    virtqueue_add_sgs
>>    virtqueue_add_outbuf
>>    virtqueue_add_inbuf
>>    virtqueue_add_inbuf_ctx
>>    virtqueue_kick_prepare
>>    virtqueue_notify
>>    virtqueue_kick
>>    virtqueue_enable_cb_prepare
>>    virtqueue_enable_cb
>>    virtqueue_enable_cb_delayed
>>    virtqueue_disable_cb
>>    virtqueue_poll
>>    virtqueue_get_buf_ctx
>>    virtqueue_get_buf
>>    virtqueue_detach_unused_buf
>>    virtqueue_get_vring_size
>>    virtqueue_resize
>>    virtqueue_is_broken
>>    virtio_break_device
>>    __virtio_unbreak_device
>>
>> Cc: Stefan Hajnoczi <stefanha@...hat.com>
>> Signed-off-by: zhenwei pi <pizhenwei@...edance.com>
>> ---
>>   drivers/virtio/virtio.c      | 362 +++++++++++++++++++++++++++++++++++
>>   drivers/virtio/virtio_ring.c | 282 +++++----------------------
>>   include/linux/virtio.h       |  29 +++
>>   3 files changed, 443 insertions(+), 230 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
>> index 3893dc29eb26..8d8715a10f26 100644
>> --- a/drivers/virtio/virtio.c
>> +++ b/drivers/virtio/virtio.c
>> @@ -553,6 +553,368 @@ int virtio_device_restore(struct virtio_device *dev)
>>   EXPORT_SYMBOL_GPL(virtio_device_restore);
>>   #endif
>>   
>> +/**
>> + * virtqueue_add_sgs - expose buffers to other end
>> + * @vq: the struct virtqueue we're talking about.
>> + * @sgs: array of terminated scatterlists.
>> + * @out_sgs: the number of scatterlists readable by other side
>> + * @in_sgs: the number of scatterlists which are writable (after readable ones)
>> + * @data: the token identifying the buffer.
>> + * @gfp: how to do memory allocations (if necessary).
>> + *
>> + * Caller must ensure we don't call this with other virtqueue operations
>> + * at the same time (except where noted).
>> + *
>> + * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
>> + */
>> +int virtqueue_add_sgs(struct virtqueue *vq, struct scatterlist *sgs[],
>> +		      unsigned int out_sgs, unsigned int in_sgs,
>> +		      void *data, gfp_t gfp)
>> +{
>> +	unsigned int i, total_sg = 0;
>> +
>> +	/* Count them first. */
>> +	for (i = 0; i < out_sgs + in_sgs; i++) {
>> +		struct scatterlist *sg;
>> +
>> +		for (sg = sgs[i]; sg; sg = sg_next(sg))
>> +			total_sg++;
>> +	}
>> +	return vq->ops->add_sgs(vq, sgs, total_sg, out_sgs, in_sgs,
>> +				data, NULL, gfp);
>> +}
>> +EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
> 
> 
> Hmm this kind of indirection on data path is going to be costly
> performance-wise especially when retpolines are in place.
> 
> Any data on this?
> 

Hi,

1, What about moving these functions into virtio.h and declare them as 
static inline?
2, what about moving method fields into struct virtqueue?

Then we have struct like:
struct virtqueue {
	struct list_head list;
	...
	void *priv;

	/* virtqueue specific operations */
         int (*add_sgs)(struct virtqueue *vq, struct scatterlist *sgs[],
                        unsigned int total_sg,
                        unsigned int out_sgs, unsigned int in_sgs,
                        void *data, void *ctx, gfp_t gfp);
	...
}

and functions like:
static inline int virtqueue_add_sgs(...)
{
         unsigned int i, total_sg = 0;

         /* Count them first. */
         for (i = 0; i < out_sgs + in_sgs; i++) {
                 struct scatterlist *sg;

                 for (sg = sgs[i]; sg; sg = sg_next(sg))
                         total_sg++;
         }
         return vq->add_sgs(vq, sgs, total_sg, out_sgs, in_sgs,
                            data, NULL, gfp);
}

If [1] is acceptable, we can also reduce changes in patch 'tools/virtio: 
implement virtqueue in test'.

-- 
zhenwei pi