Re: [PATCH v1 0/6] virtio: support advance DMA

[Jason Wang <jasowang@redhat.com>]

在 2022/2/21 下午7:23, Xuan Zhuo 写道:
> On Mon, 21 Feb 2022 11:32:52 +0800, Jason Wang <jasowang@redhat.com> wrote:
>> On Fri, Feb 18, 2022 at 5:00 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>> On Thu, 17 Feb 2022 15:19:44 +0800, Jason Wang <jasowang@redhat.com> wrote:
>>>> On Thu, Feb 10, 2022 at 4:51 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>>>> virtqueue_add() only supports virtual addresses, dma is completed in
>>>>> virtqueue_add().
>>>>>
>>>>> In some scenarios (such as the AF_XDP scenario), DMA is completed in advance, so
>>>>> it is necessary for us to support passing the DMA address to virtqueue_add().
>>>> I'd suggest rename this feature as "unmanaged DMA".
>>> OK
>>>
>>>>> Record this predma information in extra->flags, which can be skipped when
>>>>> executing dma unmap.
>>>> Question still, can we use per-virtqueue flag instead of per
>>>> descriptor flag? If my memory is correct, the answer is yes in the
>>>> discussion for the previous version.
>>>>
>>> Yes.
>>>
>>> per-virtqueue? I guess it should be per-submit.
>>>
>>> This patch set only adds a flag to desc_extra[head].flags, so that we can know
>>> if we need to unmap dma when we detach.
>> I meant if we can manage to make it per virtqueue, there's no need to
>> maintain per buffer flag.
>>
> Rethinking this question, I feel there is no essential difference between per
> virtqueue and per sgs.
>
> per virtqueue:
> 	1. add buf:
> 		a. check vq->premapped for map every sg
> 	2. detach:
> 	        a. check vq->premaped for unmap
>
> per sgs:
> 	1. add buf:
> 	        a. check function parameter "premapped" for map every sg
> 		b. add flag to extra[head].flag
>
> 	2. detach:
> 	        a: check extra[head].flag for unmap
>
>
> Thanks.


Per-virtqueue is still a little bit easier at the first glance.

Actually, per-sg have one advantage: it can be used without virtqueue 
reset (to allow switching between the two modes). But I'm not sure 
whether we had such requirements.

I think to answer this question, we probably need a real use case (if we 
can come up with a case that is more lightweight than AF_XDP, that would 
be even better).

Thanks


>
>
>> So we know something that needs to be mapped by virtio core itself,
>> e.g the indirect page. Other than this, all the rest could be
>> pre-mapped.
>>
>> For vnet header, it could be mapped by virtio-net which could be still
>> treated as pre mapped DMA since it's not the virtio ring code.
>>
>> Anything I miss here?
>>
>> Thanks
>>
>>
>>> Thanks.
>>>
>>>> Thanks
>>>>
>>>>> v1:
>>>>>     1. All sgs requested at one time are required to be unified PREDMA, and several
>>>>>        of them are not supported to be PREDMA
>>>>>     2. virtio_dma_map() is removed from this patch set and will be submitted
>>>>>        together with the next time AF_XDP supports virtio dma
>>>>>     3. Added patch #2 #3 to remove the check for flags when performing unmap
>>>>>        indirect desc
>>>>>
>>>>> Xuan Zhuo (6):
>>>>>    virtio: rename vring_unmap_state_packed() to
>>>>>      vring_unmap_extra_packed()
>>>>>    virtio: remove flags check for unmap split indirect desc
>>>>>    virtio: remove flags check for unmap packed indirect desc
>>>>>    virtio: virtqueue_add() support predma
>>>>>    virtio: split: virtqueue_add_split() support dma address
>>>>>    virtio: packed: virtqueue_add_packed() support dma address
>>>>>
>>>>>   drivers/virtio/virtio_ring.c | 199 ++++++++++++++++++++++-------------
>>>>>   1 file changed, 126 insertions(+), 73 deletions(-)
>>>>>
>>>>> --
>>>>> 2.31.0
>>>>>

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