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Date:   Thu, 31 Dec 2020 13:15:48 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.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: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb message

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.
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

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