On Thu, Jun 17, 2021 at 1:30 PM Bruce Richardson <bruce.richard...@intel.com> wrote: > > On Thu, Jun 17, 2021 at 01:12:22PM +0530, Jerin Jacob wrote: > > On Thu, Jun 17, 2021 at 12:43 AM Bruce Richardson > > <bruce.richard...@intel.com> wrote: > > > > > > On Wed, Jun 16, 2021 at 11:38:08PM +0530, Jerin Jacob wrote: > > > > On Wed, Jun 16, 2021 at 11:01 PM Bruce Richardson > > > > <bruce.richard...@intel.com> wrote: > > > > > > > > > > On Wed, Jun 16, 2021 at 05:41:45PM +0800, fengchengwen wrote: > > > > > > On 2021/6/16 0:38, Bruce Richardson wrote: > > > > > > > On Tue, Jun 15, 2021 at 09:22:07PM +0800, Chengwen Feng wrote: > > > > > > >> This patch introduces 'dmadevice' which is a generic type of DMA > > > > > > >> device. > > > > > > >> > > > > > > >> The APIs of dmadev library exposes some generic operations which > > > > > > >> can > > > > > > >> enable configuration and I/O with the DMA devices. > > > > > > >> > > > > > > >> Signed-off-by: Chengwen Feng <fengcheng...@huawei.com> > > > > > > >> --- > > > > > > > Thanks for sending this. > > > > > > > > > > > > > > Of most interest to me right now are the key data-plane APIs. > > > > > > > While we are > > > > > > > still in the prototyping phase, below is a draft of what we are > > > > > > > thinking > > > > > > > for the key enqueue/perform_ops/completed_ops APIs. > > > > > > > > > > > > > > Some key differences I note in below vs your original RFC: > > > > > > > * Use of void pointers rather than iova addresses. While using > > > > > > > iova's makes > > > > > > > sense in the general case when using hardware, in that it can > > > > > > > work with > > > > > > > both physical addresses and virtual addresses, if we change the > > > > > > > APIs to use > > > > > > > void pointers instead it will still work for DPDK in VA mode, > > > > > > > while at the > > > > > > > same time allow use of software fallbacks in error cases, and > > > > > > > also a stub > > > > > > > driver than uses memcpy in the background. Finally, using > > > > > > > iova's makes the > > > > > > > APIs a lot more awkward to use with anything but mbufs or > > > > > > > similar buffers > > > > > > > where we already have a pre-computed physical address. > > > > > > > > > > > > The iova is an hint to application, and widely used in DPDK. > > > > > > If switch to void, how to pass the address (iova or just va ?) > > > > > > this may introduce implementation dependencies here. > > > > > > > > > > > > Or always pass the va, and the driver performs address translation, > > > > > > and this > > > > > > translation may cost too much cpu I think. > > > > > > > > > > > > > > > > On the latter point, about driver doing address translation I would > > > > > agree. > > > > > However, we probably need more discussion about the use of iova vs > > > > > just > > > > > virtual addresses. My thinking on this is that if we specify the API > > > > > using > > > > > iovas it will severely hurt usability of the API, since it forces the > > > > > user > > > > > to take more inefficient codepaths in a large number of cases. Given a > > > > > pointer to the middle of an mbuf, one cannot just pass that straight > > > > > as an > > > > > iova but must instead do a translation into offset from mbuf pointer > > > > > and > > > > > then readd the offset to the mbuf base address. > > > > > > > > > > My preference therefore is to require the use of an IOMMU when using a > > > > > dmadev, so that it can be a much closer analog of memcpy. Once an > > > > > iommu is > > > > > present, DPDK will run in VA mode, allowing virtual addresses to our > > > > > hugepage memory to be sent directly to hardware. Also, when using > > > > > dmadevs on top of an in-kernel driver, that kernel driver may do all > > > > > iommu > > > > > management for the app, removing further the restrictions on what > > > > > memory > > > > > can be addressed by hardware. > > > > > > > > > > > > One issue of keeping void * is that memory can come from stack or heap . > > > > which HW can not really operate it on. > > > > > > when kernel driver is managing the IOMMU all process memory can be worked > > > on, not just hugepage memory, so using iova is wrong in these cases. > > > > But not for stack and heap memory. Right? > > > Yes, even stack and heap can be accessed.
The HW device cannot as that memory is NOT mapped to IOMMU. It will result in the transaction fault. At least, In octeon, DMA HW job descriptor will have a pointer (IOVA) which will be updated by _HW_ upon copy job completion. That memory can not be from the heap(malloc()) or stack as those are not mapped by IOMMU. > > > > > > > As I previously said, using iova prevents the creation of a pure software > > > dummy driver too using memcpy in the background. > > > > Why ? the memory alloced uing rte_alloc/rte_memzone etc can be touched by > > CPU. > > > Yes, but it can't be accessed using physical address, so again only VA mode > where iova's are "void *" make sense. I agree that it should be a physical address. My only concern that void * does not express it can not be from stack/heap. If API tells the memory need to allotted by rte_alloc() or rte_memzone() etc is fine with me. or it may better that. Have separate API to alloc the handle so based on the driver, it can be rte_alloc() or malloc(). It can be burst API in slow path to get number of status pointers > > > Thinking more, Since anyway, we need a separate function for knowing > > the completion status, > > I think, it can be an opaque object as the completion code. Exposing > > directly the status may not help > > . As the driver needs a "context" or "call" to change the > > driver-specific completion code to DPDK completion code. > > > I'm sorry, I didn't follow this. By completion code, you mean the status of > whether a copy job succeeded/failed? Yes, the status of job completion.
