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

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Date:   Thu, 31 Dec 2020 15:11:46 +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2:52, Yongji Xie wrote:
> On Thu, Dec 31, 2020 at 1:50 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 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?
>>
> Yes, it might be hard to reuse the memory. For example, we firstly
> allocate 1 IOVA/page during dma_map, then the IOVA is freed during
> dma_unmap. Actually we can't reuse this single page if we need a
> two-pages area in the next IOVA allocating. So the best way is using
> IOTLB_UNMAP to free this single page during dma_unmap too.
>
> Thanks,
> Yongji


I get you now. Then I agree that let's go with IOTLB_UNMAP.

Thanks


>