linux-kernel - Re: Re: [PATCH v2 1/2] virtio: abstract virtqueue related methods

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Date:   Wed, 17 May 2023 16:35:55 +0800
From:   zhenwei pi <pizhenwei@...edance.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     stefanha@...hat.com, mst@...hat.com, jasowang@...hat.com,
        xuanzhuo@...ux.alibaba.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH v2 1/2] virtio: abstract virtqueue related methods



On 5/17/23 15:46, Christoph Hellwig wrote:
> On Wed, May 17, 2023 at 03:43:03PM +0800, zhenwei pi wrote:
>> I have a plan to introduce 'Virtio Over Fabrics'(TCP&RDMA) as Virtio
>> transport, as mentioned in cover letter of this series:
>> 3 weeks ago, I posted a proposal 'Virtio Over Fabrics':
>> https://lists.oasis-open.org/archives/virtio-comment/202304/msg00442.html
> 
> Just don't do it.  Please define your own protocols over RDMA or TCP
> for exactly the operations you need (for many they will already exist)
> instead of piggyg backing on virtio and making everyone else pay the
> price.
> 

Hi

1, `virtqueue_add_inbuf` in current version:
static inline int virtqueue_add_inbuf(struct virtqueue *vq,
                                       struct scatterlist *sg,
                                       unsigned int num,
                                       void *data,
                                       gfp_t gfp)
{
         if (likely(!vq->abstract))
                 return vring_virtqueue_add_sgs(vq, &sg, num, 0, 1, 
data, NULL, gfp);

         return vq->add_sgs(vq, &sg, num, 0, 1, data, NULL, gfp);
}

And disassemble 'virtinput_queue_evtbuf':
static void virtinput_queue_evtbuf(struct virtio_input *vi,
                                    struct virtio_input_event *evtbuf)
{
         struct scatterlist sg[1];

         sg_init_one(sg, evtbuf, sizeof(*evtbuf));
         virtqueue_add_inbuf(vi->evt, sg, 1, evtbuf, GFP_ATOMIC);
}

I notice that two instructions are newly added for vring like:
  24d:   80 78 35 00             cmpb   $0x0,0x35(%rax)
  251:   75 3f                   jne    292

Is it an expensive price...

2, Storage/FS specific remote protocol is quite popular, otherwise I'm 
not familiar with other device protocols. For example, I need a remote 
crypto device to accelerate HTTPS ... With Virtio Over Fabrics, I have a 
chance to attach a virtio-crypto device to do this work.

-- 
zhenwei pi