Re: [RFC PATCH v4 0/2] RDMA/rxe: Add dma-buf support

[Shunsuke Mie]

2021年12月14日(火) 17:54 Christian König <christian.koenig@xxxxxxx>:
>
> Am 13.12.21 um 12:18 schrieb Shunsuke Mie:
> > 2021年12月10日(金) 22:29 Christian König <christian.koenig@xxxxxxx>:
> >> Am 10.12.21 um 14:26 schrieb Jason Gunthorpe:
> >>> On Fri, Dec 10, 2021 at 01:47:37PM +0100, Christian König wrote:
> >>>> Am 10.12.21 um 13:42 schrieb Jason Gunthorpe:
> >>>>> On Fri, Dec 10, 2021 at 08:29:24PM +0900, Shunsuke Mie wrote:
> >>>>>> Hi Jason,
> >>>>>> Thank you for replying.
> >>>>>>
> >>>>>> 2021年12月8日(水) 2:14 Jason Gunthorpe <jgg@xxxxxxxx>:
> >>>>>>> On Fri, Dec 03, 2021 at 12:51:44PM +0900, Shunsuke Mie wrote:
> >>>>>>>> Hi maintainers,
> >>>>>>>>
> >>>>>>>> Could you please review this patch series?
> >>>>>>> Why is it RFC?
> >>>>>>>
> >>>>>>> I'm confused why this is useful?
> >>>>>>>
> >>>>>>> This can't do copy from MMIO memory, so it shouldn't be compatible
> >>>>>>> with things like Gaudi - does something prevent this?
> >>>>>> I think if an export of the dma-buf supports vmap, CPU is able to access the
> >>>>>> mmio memory.
> >>>>>>
> >>>>>> Is it wrong? If this is wrong, there is no advantages this changes..
> >>>>> I don't know what the dmabuf folks did, but yes, it is wrong.
> >>>>>
> >>>>> IOMEM must be touched using only special accessors, some platforms
> >>>>> crash if you don't do this. Even x86 will crash if you touch it with
> >>>>> something like an XMM optimized memcpy.
> >>>>>
> >>>>> Christian? If the vmap succeeds what rules must the caller use to
> >>>>> access the memory?
> >>>> See dma-buf-map.h and especially struct dma_buf_map.
> >>>>
> >>>> MMIO memory is perfectly supported here and actually the most common case.
> >>> Okay that looks sane, but this rxe RFC seems to ignore this
> >>> completely. It stuffs the vaddr directly into a umem which goes to all
> >>> manner of places in the driver.
> >>>
> >>> ??
> >> Well, yes that can go boom pretty quickly.
> > Sorry, I was wrong. The dma_buf_map treats both iomem and vaddr region, but
> > this RFC only supports vaddr. Advantage of the partial support is we can use the
> > vaddr dma-buf in RXE without changing a rxe data copy implementation.
>
> Well that is most likely not a good idea.
>
> For example buffers for GPU drivers can be placed in both MMIO memory
> and system memory.
>
> If you don't want to provoke random failures you *MUST* be able to
> handle both if you want to use this.
I agree with you. I'll add the support and resubmit patch series.

Thanks a lot,
Shunsuke.
>
> Regards,
> Christian.
>
> >
> > An example of a dma-buf pointing to a vaddr is some gpu drivers use RAM for
> > VRAM and we can get dma-buf for the region that indicates vaddr regions.
> > Specifically, the gpu driver using gpu/drm/drm_gem_cma_helper.c is one such
> > example.
> >
> >> Not sure what they want to use this for.
> > I'd like to use RDMA with RXE for that memory region.
> >
> > Best,
> > Shunsuke
> >> Christian.
> >>
> >>> Jason
>