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

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Date:   Wed, 13 Jun 2018 23:11:01 +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 Wed, 2018-06-13 at 22:25 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2018-06-13 at 00:41 -0700, Christoph Hellwig wrote:
> > 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.
> 
> By direct ops I didn't mean *the* dma_direct_ops but a virtio-local
> variants that effectively reproduces the existing expectations (ie,
> virtio-dma-legacy-ops or something).

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 ?

Cheers,
Ben.
> 
> > 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.
> 
> Ben.