linux-kernel - Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

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Date:   Thu, 7 Oct 2021 08:59:18 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Barry Song <21cnbao@...il.com>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>, mike.campin@...el.com,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

On Fri, Oct 08, 2021 at 12:54:52AM +1300, Barry Song wrote:
> On Fri, Oct 8, 2021 at 12:32 AM Jason Gunthorpe <jgg@...dia.com> wrote:
> >
> > On Thu, Oct 07, 2021 at 06:43:33PM +1300, Barry Song wrote:
> >
> > > So do we have a case where devices can directly access the kernel's data
> > > structure such as a list/graph/tree with pointers to a kernel virtual address?
> > > then devices don't need to translate the address of pointers in a structure.
> > > I assume this is one of the most useful features userspace SVA can provide.
> >
> > AFIACT that is the only good case for KVA, but it is also completely
> > against the endianess, word size and DMA portability design of the
> > kernel.
> >
> > Going there requires some new set of portable APIs for gobally
> > coherent KVA dma.
> 
> yep. I agree. it would be very weird if accelerators/gpu are sharing
> kernel' data struct, but for each "DMA" operation - reading or writing
> the data struct, we have to call dma_map_single/sg or
> dma_sync_single_for_cpu/device etc. It seems once devices and cpus
> are sharing virtual address(SVA), code doesn't need to do explicit
> map/sync each time.

No, it still need to do something to manage visibility from the
current CPU to the DMA - it might not be flushing a cache, but it is
probably a arch specific CPU barrier instruction.

Jason