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

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Message-ID: <20180805032504-mutt-send-email-mst@kernel.org>
Date:   Sun, 5 Aug 2018 03:27:42 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christoph Hellwig <hch@...radead.org>,
        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 Wed, Aug 01, 2018 at 09:16:38AM +0100, Will Deacon wrote:
> On Tue, Jul 31, 2018 at 03:36:22PM -0500, Benjamin Herrenschmidt wrote:
> > On Tue, 2018-07-31 at 10:30 -0700, Christoph Hellwig wrote:
> > > > However the question people raise is that DMA API is already full of
> > > > arch-specific tricks the likes of which are outlined in your post linked
> > > > above. How is this one much worse?
> > > 
> > > None of these warts is visible to the driver, they are all handled in
> > > the architecture (possibly on a per-bus basis).
> > > 
> > > So for virtio we really need to decide if it has one set of behavior
> > > as specified in the virtio spec, or if it behaves exactly as if it
> > > was on a PCI bus, or in fact probably both as you lined up.  But no
> > > magic arch specific behavior inbetween.
> > 
> > The only arch specific behaviour is needed in the case where it doesn't
> > behave like PCI. In this case, the PCI DMA ops are not suitable, but in
> > our secure VMs, we still need to make it use swiotlb in order to bounce
> > through non-secure pages.
> 
> On arm/arm64, the problem we have is that legacy virtio devices on the MMIO
> transport (so definitely not PCI) have historically been advertised by qemu
> as not being cache coherent, but because the virtio core has bypassed DMA
> ops then everything has happened to work. If we blindly enable the arch DMA
> ops, we'll plumb in the non-coherent ops and start getting data corruption,
> so we do need a way to quirk virtio as being "always coherent" if we want to
> use the DMA ops (which we do, because our emulation platforms have an IOMMU
> for all virtio devices).
> 
> Will

Right that's not very different from placing the device within the IOMMU
domain but in fact bypassing the IOMMU. I wonder whether anyone ever
needs a non coherent virtio-mmio. If yes we can extend
PLATFORM_IOMMU to cover that or add another bit.

What exactly do the non-coherent ops do that causes the corruption?

-- 
MST