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

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Date:   Thu, 31 Dec 2020 13:49:59 +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 2020/12/31 下午1:15, Yongji Xie wrote:
> On Thu, Dec 31, 2020 at 10:49 AM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 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.
>>
> Yes, that's true.
>
>> 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?
>>
> In coherent DMA cases, we need to return some memory with contiguous
> kernel virtual addresses. That is the reason why we need vmalloc()
> here. If we allocate the memory page by page, the corresponding kernel
> virtual addresses in a contiguous IOVA range might not be contiguous.


Yes, but we can do that as what has been done in the series 
(alloc_pages_exact()). Or do you mean it would be a little bit hard to 
recycle IOVA/pages here?

Thanks


> And in streaming DMA cases, there is no limit. So another choice is
> using vmalloc'ed memory only for coherent DMA cases.
>
> Not sure if this is clear for you.
>
> Thanks,
> Yongji
>