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

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Message-ID: <CAGsJ_4x2UEmNXCVhJAVRyB8568VMrTkOLeVCNp8CyP6xZJwCig@mail.gmail.com>
Date:   Fri, 8 Oct 2021 00:54:52 +1300
From:   Barry Song <21cnbao@...il.com>
To:     Jason Gunthorpe <jgg@...dia.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 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.

>
> Jason

Thanks
barry