linux-kernel - Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices

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Message-ID: <1b4732c5015ee4377fef1de0270065174cde397a.camel@kernel.crashing.org>
Date:   Sat, 16 Jun 2018 11:07:24 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Ram Pai <linuxram@...ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>, robh@...nel.org,
        pawel.moll@....com, Tom Lendacky <thomas.lendacky@....com>,
        aik@...abs.ru, jasowang@...hat.com, cohuck@...hat.com,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, joe@...ches.com,
        "Rustad, Mark D" <mark.d.rustad@...el.com>,
        david@...son.dropbear.id.au, linuxppc-dev@...ts.ozlabs.org,
        elfring@...rs.sourceforge.net,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Subject: Re: [RFC V2] virtio: Add platform specific DMA API translation for
 virito devices

On Fri, 2018-06-15 at 02:16 -0700, Christoph Hellwig wrote:
> On Wed, Jun 13, 2018 at 11:11:01PM +1000, Benjamin Herrenschmidt wrote:
> > Actually ... the stuff in lib/dma-direct.c seems to be just it, no ?
> > 
> > There's no cache flushing and there's no architecture hooks that I can
> > see other than the AMD security stuff which is probably fine.
> > 
> > Or am I missing something ?
> 
> You are missing the __phys_to_dma arch hook that allows architectures
> to adjust the dma address.  Various systems have offsets, or even
> multiple banks with different offsets there.  Most of them don't
> use the dma-direct code yet (working on it), but there are a few
> examples in the tree already.

Ok and on those systems, qemu will bypass said offset ? Maybe we could
just create a device DMA "flag" or (or use the attributes) to instruct
dma-direct to not use that for legacy virtio ?

Cheers,
Ben.