On Tue, 22 Feb 2022 12:02:14 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote: > > 在 2022/2/21 下午7:23, Xuan Zhuo 写道: > > On Mon, 21 Feb 2022 11:32:52 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote: > >> On Fri, Feb 18, 2022 at 5:00 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote: > >>> On Thu, 17 Feb 2022 15:19:44 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote: > >>>> On Thu, Feb 10, 2022 at 4:51 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote: > >>>>> virtqueue_add() only supports virtual addresses, dma is completed in > >>>>> virtqueue_add(). > >>>>> > >>>>> In some scenarios (such as the AF_XDP scenario), DMA is completed in advance, so > >>>>> it is necessary for us to support passing the DMA address to virtqueue_add(). > >>>> I'd suggest rename this feature as "unmanaged DMA". > >>> OK > >>> > >>>>> Record this predma information in extra->flags, which can be skipped when > >>>>> executing dma unmap. > >>>> Question still, can we use per-virtqueue flag instead of per > >>>> descriptor flag? If my memory is correct, the answer is yes in the > >>>> discussion for the previous version. > >>>> > >>> Yes. > >>> > >>> per-virtqueue? I guess it should be per-submit. > >>> > >>> This patch set only adds a flag to desc_extra[head].flags, so that we can know > >>> if we need to unmap dma when we detach. > >> I meant if we can manage to make it per virtqueue, there's no need to > >> maintain per buffer flag. > >> > > Rethinking this question, I feel there is no essential difference between per > > virtqueue and per sgs. > > > > per virtqueue: > > 1. add buf: > > a. check vq->premapped for map every sg > > 2. detach: > > a. check vq->premaped for unmap > > > > per sgs: > > 1. add buf: > > a. check function parameter "premapped" for map every sg > > b. add flag to extra[head].flag > > > > 2. detach: > > a: check extra[head].flag for unmap > > > > > > Thanks. > > > Per-virtqueue is still a little bit easier at the first glance. > > Actually, per-sg have one advantage: it can be used without virtqueue > reset (to allow switching between the two modes). But I'm not sure > whether we had such requirements. > > I think to answer this question, we probably need a real use case (if we > can come up with a case that is more lightweight than AF_XDP, that would > be even better). Sadly, I didn't think of other scenarios. Hope someone can give a scenario. For per virtqueue, virtio-net will also switch to premapped. Because the tx queue is shared. But in the process of implementing this, I encountered a troublesome problem. We need to record the dma address in virtnet. For tx, since skb contains multiple frags, there will be many dma addresses. When unmap in virtnet It will be more troublesome. Because we have to regain these dma addresses. I think of two ways: 1. Let virtio return the addr of each desc when detached. 2. Allocate a block of memory for each sq/rq to hold the dma address. Thanks. > > Thanks > > > > > > > >> So we know something that needs to be mapped by virtio core itself, > >> e.g the indirect page. Other than this, all the rest could be > >> pre-mapped. > >> > >> For vnet header, it could be mapped by virtio-net which could be still > >> treated as pre mapped DMA since it's not the virtio ring code. > >> > >> Anything I miss here? > >> > >> Thanks > >> > >> > >>> Thanks. > >>> > >>>> Thanks > >>>> > >>>>> v1: > >>>>> 1. All sgs requested at one time are required to be unified PREDMA, and several > >>>>> of them are not supported to be PREDMA > >>>>> 2. virtio_dma_map() is removed from this patch set and will be submitted > >>>>> together with the next time AF_XDP supports virtio dma > >>>>> 3. Added patch #2 #3 to remove the check for flags when performing unmap > >>>>> indirect desc > >>>>> > >>>>> Xuan Zhuo (6): > >>>>> virtio: rename vring_unmap_state_packed() to > >>>>> vring_unmap_extra_packed() > >>>>> virtio: remove flags check for unmap split indirect desc > >>>>> virtio: remove flags check for unmap packed indirect desc > >>>>> virtio: virtqueue_add() support predma > >>>>> virtio: split: virtqueue_add_split() support dma address > >>>>> virtio: packed: virtqueue_add_packed() support dma address > >>>>> > >>>>> drivers/virtio/virtio_ring.c | 199 ++++++++++++++++++++++------------- > >>>>> 1 file changed, 126 insertions(+), 73 deletions(-) > >>>>> > >>>>> -- > >>>>> 2.31.0 > >>>>> > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
