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

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Date:   Mon, 6 Aug 2018 23:21:17 -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 Tue, Aug 07, 2018 at 05:52:12AM +1000, Benjamin Herrenschmidt wrote:
> > It is your job to write a coherent interface specification that does
> > not depend on the used components.  The hypervisor might be PAPR,
> > Linux + qemu, VMware, Hyperv or something so secret that you'd have
> > to shoot me if you had to tell me.  The guest might be Linux, FreeBSD,
> > AIX, OS400 or a Hipster project of the day in Rust.  As long as we
> > properly specify the interface it simplify does not matter.
> 
> That's the point Christoph. The interface is today's interface. It does
> NOT change. That information is not part of the interface.
> 
> It's the VM itself that is stashing away its memory in a secret place,
> and thus needs to do bounce buffering. There is no change to the virtio
> interface per-se.

Any guest that doesn't know about your magic limited adressing is simply
not going to work, so we need to communicate that fact.