On Tue, Apr 25, 2023 at 04:21:53PM +0800, Xuan Zhuo wrote: > On Tue, 25 Apr 2023 04:13:09 -0400, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > On Tue, Apr 25, 2023 at 04:05:03PM +0800, Xuan Zhuo wrote: > > > On Tue, 25 Apr 2023 03:51:47 -0400, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > On Tue, Apr 25, 2023 at 03:36:02PM +0800, Xuan Zhuo wrote: > > > > > ## About DMA APIs > > > > > > > > > > Now, virtio may can not work with DMA APIs when virtio features do not have > > > > > VIRTIO_F_ACCESS_PLATFORM. > > > > > > > > > > 1. I tried to let DMA APIs return phy address by virtio-device. But DMA APIs just > > > > > work with the "real" devices. > > > > > 2. I tried to let xsk support callballs to get phy address from virtio-net > > > > > driver as the dma address. But the maintainers of xsk may want to use dma-buf > > > > > to replace the DMA APIs. I think that may be a larger effort. We will wait > > > > > too long. > > > > > > > > > > So rethinking this, firstly, we can support premapped-dma only for devices with > > > > > VIRTIO_F_ACCESS_PLATFORM. In the case of af-xdp, if the users want to use it, > > > > > they have to update the device to support VIRTIO_F_RING_RESET, and they can also > > > > > enable the device's VIRTIO_F_ACCESS_PLATFORM feature by the way. > > > > > > > > I don't understand this last sentence. If you think ring > > > > reset can change device features then the answer is no, it can't. > > > > > > > > > Sorry, I should remove "by the way". > > > > > > > > > > > > > > If you are saying device has to set VIRTIO_F_ACCESS_PLATFORM to > > > > benefit from this work, that's fine at least as a first approach. > > > > Note that setting VIRTIO_F_ACCESS_PLATFORM breaks old guests > > > > (it's a secirity boundary), e.g. it is not available for > > > > transitional devices. > > > > So to support transitional devices, we might want to find another way to > > > > address this down the road, > > > > > > Maybe dma-buf is a way. I'll look into it, especially some practice on xsk. > > > > > > > but as a first step, I agree just going with > > > > DMA is fine. > > > > > > > > > Thanks. > > > > Pls do make sure to disable the feature when !VIRTIO_F_ACCESS_PLATFORM > > though. > > If you refer to the implementation inside virtio-net, this feature will depend > on the return of virtqueue_dma_dev(). > > But virtqueue_dma_dev() depends "use_dma_api". When xen_domain() is true and > !VIRTIO_F_ACCESS_PLATFORM, the "use_dma_api" is true. > > So what kind of situation do you mean? > > Thanks. E.g. a legacy device. -- MST
