Date:   Wed, 30 Dec 2020 16:41:30 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, sgarzare@...hat.com,
        Parav Pandit <parav@...dia.com>, akpm@...ux-foundation.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        axboe@...nel.dk, bcrl@...ck.org, corbet@....net,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb
 message


On 2020/12/30 下午3:09, Yongji Xie wrote:
> On Wed, Dec 30, 2020 at 2:11 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> On 2020/12/29 下午6:26, Yongji Xie wrote:
>>> On Tue, Dec 29, 2020 at 5:11 PM Jason Wang <jasowang@...hat.com> wrote:
>>>>
>>>> ----- Original Message -----
>>>>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang <jasowang@...hat.com> wrote:
>>>>>> On 2020/12/28 下午4:14, Yongji Xie wrote:
>>>>>>>> I see. So all the above two questions are because VHOST_IOTLB_INVALIDATE
>>>>>>>> is expected to be synchronous. This need to be solved by tweaking the
>>>>>>>> current VDUSE API or we can re-visit to go with descriptors relaying
>>>>>>>> first.
>>>>>>>>
>>>>>>> Actually all vdpa related operations are synchronous in current
>>>>>>> implementation. The ops.set_map/dma_map/dma_unmap should not return
>>>>>>> until the VDUSE_UPDATE_IOTLB/VDUSE_INVALIDATE_IOTLB message is replied
>>>>>>> by userspace. Could it solve this problem?
>>>>>>     I was thinking whether or not we need to generate IOTLB_INVALIDATE
>>>>>> message to VDUSE during dma_unmap (vduse_dev_unmap_page).
>>>>>>
>>>>>> If we don't, we're probably fine.
>>>>>>
>>>>> It seems not feasible. This message will be also used in the
>>>>> virtio-vdpa case to notify userspace to unmap some pages during
>>>>> consistent dma unmapping. Maybe we can document it to make sure the
>>>>> users can handle the message correctly.
>>>> Just to make sure I understand your point.
>>>>
>>>> Do you mean you plan to notify the unmap of 1) streaming DMA or 2)
>>>> coherent DMA?
>>>>
>>>> For 1) you probably need a workqueue to do that since dma unmap can
>>>> be done in irq or bh context. And if usrspace does't do the unmap, it
>>>> can still access the bounce buffer (if you don't zap pte)?
>>>>
>>> I plan to do it in the coherent DMA case.
>>
>> Any reason for treating coherent DMA differently?
>>
> Now the memory of the bounce buffer is allocated page by page in the
> page fault handler. So it can't be used in coherent DMA mapping case
> which needs some memory with contiguous virtual addresses. I can use
> vmalloc() to do allocation for the bounce buffer instead. But it might
> cause some memory waste. Any suggestion?


I may miss something. But I don't see a relationship between the 
IOTLB_UNMAP and vmalloc().


>
>>> It's true that userspace can
>>> access the dma buffer if userspace doesn't do the unmap. But the dma
>>> pages would not be freed and reused unless user space called munmap()
>>> for them.
>>
>> I wonder whether or not we could recycle IOVA in this case to avoid the
>> IOTLB_UMAP message.
>>
> We can achieve that if we use vmalloc() to do allocation for the
> bounce buffer which can be used in coherent DMA mapping case. But
> looks like we still have no way to avoid the IOTLB_UMAP message in
> vhost-vdpa case.


I think that's fine. For virtio-vdpa, from VDUSE userspace perspective, 
it works like a driver that is using SWIOTLB in this case.

Thanks


>
> Thanks,
> Yongji
>

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