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Date:   Thu, 31 Dec 2020 10:49:33 +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6:12, Yongji Xie wrote:
> On Wed, Dec 30, 2020 at 4:41 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 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().
>>
> In the vmalloc() case, the coherent DMA page will be taken from the
> memory allocated by vmalloc(). So IOTLB_UNMAP is not needed anymore
> during coherent DMA unmapping because those vmalloc'ed memory which
> has been mapped into userspace address space during initialization can
> be reused. And userspace should not unmap the region until we destroy
> the device.


Just to make sure I understand. My understanding is that IOTLB_UNMAP is 
only needed when there's a change the mapping from IOVA to page.

So if we stick to the mapping, e.g during dma_unmap, we just put IOVA to 
free list to be used by the next IOVA allocating. IOTLB_UNMAP could be 
avoided.

So we are not limited by how the pages are actually allocated?

Thanks


>
>>>>> 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.
>>
> OK, will do it in v3.
>
> Thanks,
> Yongji
>

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