On Fri, Feb 21, 2020 at 05:36:45PM +0100, Christoph Hellwig wrote:

On Fri, Feb 21, 2020 at 01:59:15PM +1100, David Gibson wrote:

quoted

quoted

Hell no.  This is a detail of the platform DMA direct implementation.
Drivers have no business looking at this flag, and virtio finally needs
to be fixed to use the DMA API properly for everything but legacy devices.

So, this patch definitely isn't right as it stands, but I'm struggling
to understand what it is you're saying is the right way.

By "legacy devices" I assume you mean pre-virtio-1.0 devices, that
lack the F_VERSION_1 feature flag.  Is that right?  Because I don't
see how being a legacy device or not relates to use of the DMA API.

No.   "legacy" is anything that does not set F_ACCESS_PLATFORM.

Hm, I see.

The trouble is I think we can only reasonably call things "legacy"
when essentially all currently in use OSes have support for the new,
better way of doing things.  That is, alas, not really the case for
F_ACCESS_PLATFORM.

quoted

I *think* what you are suggesting here is that virtio devices that
have !F_IOMMU_PLATFORM should have their dma_ops set up so that the
DMA API treats IOVA==PA, which will satisfy what the device expects.
Then the virtio driver can use the DMA API the same way for both
F_IOMMU_PLATFORM and !F_IOMMU_PLATFORM devices.

No.  Those should just keep using the existing legacy non-dma ops
case and be done with it.  No changes to that and most certainly
no new features.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson