Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb message

From: Jason Wang <jasowang@redhat.com>
To: Yongji Xie <xieyongji@bytedance.com>
Cc: axboe@kernel.dk, corbet@lwn.net, kvm@vger.kernel.org,
	"Michael S. Tsirkin" <mst@redhat.com>,
	linux-aio@kvack.org, netdev@vger.kernel.org,
	Randy Dunlap <rdunlap@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	virtualization@lists.linux-foundation.org, linux-mm@kvack.org,
	bcrl@kvack.org, viro@zeniv.linux.org.uk,
	Stefan Hajnoczi <stefanha@redhat.com>,
	linux-fsdevel@vger.kernel.org, akpm@linux-foundation.org
Subject: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb message
Date: Wed, 30 Dec 2020 14:10:57 +0800	[thread overview]
Message-ID: <b1aef426-29c7-7244-5fc9-56d52e86abb4@redhat.com> (raw)
In-Reply-To: <CACycT3sg61yRdupnD+jQEkWKsVEvMWfhkJ=5z_bYZLxCibDiHw@mail.gmail.com>

On 2020/12/29 下午6:26, Yongji Xie wrote:
> On Tue, Dec 29, 2020 at 5:11 PM Jason Wang <jasowang@redhat.com> wrote:
>>
>>
>> ----- Original Message -----
>>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang <jasowang@redhat.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?

> 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.

Thanks

>
> Thanks,
> Yongji
>

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