linux-kernel - Re: [RFC 0/4] Virtio uses DMA API for all devices

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Date:   Sat, 4 Aug 2018 01:21:20 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        aik@...abs.ru, robh@...nel.org, joe@...ches.com,
        elfring@...rs.sourceforge.net, david@...son.dropbear.id.au,
        jasowang@...hat.com, mpe@...erman.id.au, linuxram@...ibm.com,
        haren@...ux.vnet.ibm.com, paulus@...ba.org,
        srikar@...ux.vnet.ibm.com, robin.murphy@....com,
        jean-philippe.brucker@....com, marc.zyngier@....com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices

On Fri, Aug 03, 2018 at 01:58:46PM -0500, Benjamin Herrenschmidt wrote:
> You are saying something along the lines of "I don't like an
> instruction in your ISA, let's not support your entire CPU architecture
> in Linux".

No.  I'm saying if you can't describe your architecture in the virtio
spec document it is bogus.

> Our setup is not fucked. It makes a LOT of sense and it's a very
> sensible design. It's hitting a problem due to a corner case oddity in
> virtio bypassing the MMU, we've worked around such corner cases many
> times in the past without any problem, I fail to see what the problem
> is here.

No matter if you like it or not (I don't!) virtio is defined to bypass
dma translations, it is very clearly stated in the spec.  It has some
ill-defined bits to bypass it, so if you want the dma mapping API
to be used you'll have to set that bit (in its original form, a refined
form, or an entirely newly defined sane form) and make sure your
hypersivors always sets it.  It's not rocket science, just a little bit
for work to make sure your setup is actually going to work reliably
and portably.

> We aren't going to cancel years of HW and SW development for our

Maybe you should have actually read the specs you are claiming to
implemented before spending all that effort.