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

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Date:   Wed, 17 May 2023 06:39:43 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     zhenwei pi <pizhenwei@...edance.com>
Cc:     Christoph Hellwig <hch@...radead.org>, stefanha@...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 Wed, May 17, 2023 at 04:35:55PM +0800, zhenwei pi wrote:
> 
> 
> 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...

Can we somehow only override the kick method?
Then take the ring and send it over ...


> 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