Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices

From: Will Deacon <[email-masked]>
Date: Wed, 1 Aug 2018 10:05:35 +0100

Hi Christoph,

On Wed, Aug 01, 2018 at 01:36:39AM -0700, Christoph Hellwig wrote:
> On Wed, Aug 01, 2018 at 09:16:38AM +0100, Will Deacon wrote:
> > 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,
> 
> No one is suggesting that as far as I can tell.

Apologies: it's me that wants the DMA ops enabled to handle legacy devices
behind an IOMMU, but see below.

> > 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).
> 
> From all that I've gather so far: no you do not want that.  We really
> need to figure out virtio "dma" interacts with the host / device.
> 
> If you look at the current iommu spec it does talk of physical address
> with a little careveout for VIRTIO_F_IOMMU_PLATFORM.

That's true, although that doesn't exist in the legacy virtio spec, and we
have an existing emulation platform which puts legacy virtio devices behind
an IOMMU. Currently, Linux is unable to boot on this platform unless the
IOMMU is configured as bypass. If we can use the coherent IOMMU DMA ops,
then it works perfectly.

> So between that and our discussion in this thread and its previous
> iterations I think we need to stick to the current always physical,
> bypass system dma ops mode of virtio operation as the default.

As above -- that means we hang during boot because we get stuck trying to
bring up a virtio-block device whose DMA is aborted by the IOMMU. The easy
answer is "just upgrade to latest virtio and advertise the presence of the
IOMMU". I'm pushing for that in future platforms, but it seems a shame not
to support the current platform, especially given that other systems do have
hacks in mainline to get virtio working.

> We just need to figure out how to deal with devices that deviate
> from the default.  One things is that VIRTIO_F_IOMMU_PLATFORM really
> should become VIRTIO_F_PLATFORM_DMA to cover the cases of non-iommu
> dma tweaks (offsets, cache flushing), which seems well in spirit of
> the original design.  The other issue is VIRTIO_F_IO_BARRIER
> which is very vaguely defined, and which needs a better definition.
> And last but not least we'll need some text explaining the challenges
> of hardware devices - I think VIRTIO_F_PLATFORM_DMA + VIRTIO_F_IO_BARRIER
> is what would basically cover them, but a good description including
> an explanation of why these matter.

I agree that this makes sense for future revisions of virtio (or perhaps
it can just be a clarification to virtio 1.0), but we're still left in the
dark with legacy devices and it would be nice to have them work on the
systems which currently exist, even if it's a legacy-only hack in the arch
code.

Will