Date:   Thu, 31 Dec 2020 16:00:38 +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 3:12 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> 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.
>

Fine, will do it.

Thanks,
Yongji

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