On Fri, 21 Feb 2020 17:36:45 +0100
Christoph Hellwig [off-list ref] wrote:

quoted

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.

FYI in virtio-speak the term 'legacy devices' is already taken and it
ain't congruent with your intended semantics. Please look it up
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-60001

But with that understood your statement does provide insisting in how do
you see F_ACCESS_PLATFORM. I.e. something that should be enabled in
general, except for legacy reasons. But I'm afraid Michael sees it
differently: i.e. something that should be enabled when necessary, and
otherwise not (because it is likely to cost performance).

Regards,
Halil