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

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Message-ID: <20180613074141.GA12033@infradead.org>
Date:   Wed, 13 Jun 2018 00:41:41 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Ram Pai <linuxram@...ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Christoph Hellwig <hch@...radead.org>, 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 Mon, Jun 11, 2018 at 01:29:18PM +1000, Benjamin Herrenschmidt wrote:
> At the risk of repeating myself, let's just do the first pass which is
> to switch virtio over to always using the DMA API in the actual data
> flow code, with a hook at initialization time that replaces the DMA ops
> with some home cooked "direct" ops in the case where the IOMMU flag
> isn't set.
> 
> This will be equivalent to what we have today but avoids having 2
> separate code path all over the driver.
> 
> Then a second stage, I think, is to replace this "hook" so that the
> architecture gets a say in the matter.

I don't think we can actually use dma_direct_ops.  It still allows
architectures to override parts of the dma setup, which virtio seems
to blindly assume phys == dma and not cache flushing.

I think the right way forward is to either add a new
VIRTIO_F_IS_PCI_DEVICE (or redefine the existing iommu flag if deemed
possible).  And then make sure recent qemu always sets it.