Re: [dpdk-dev] [RFC PATCH] dmadev: introduce DMA device library

[Bruce Richardson <bruce.richardson@intel.com>]

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.richardson@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.richardson@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 <fengchengwen@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.

> >
> > 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.

> 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?