On 11/03/2009 12:33 AM, Anthony Liguori wrote:
Michael S. Tsirkin wrote:
devices should have the final say over which virtio features they
support. E.g. indirect entries may or may not make sense in the context
of virtio-console. In particular, for vhost, we do not want to report to
guest bits not supported by kernel backend. Move the common bits from
virtio-pci to an inline function and let each device call it.
No functional changes.
This is a layering violation. There are transport specific features
and device specific features. The virtio-net device should have no
knowledge or nack'ing ability for transport features.
It's equivalent to -cpu host. Sometimes you want to pass-through host
capabilities in order to make the best use of your hardware. In fact,
even -cpu !host allows the host kernel to nack features since the cost
of emulation is prohibitive.
If you need to change transport features, it suggests you're modeling
things incorrectly and should be supplying an alternative transport
implementation.
Since the kernel and qemu are developed independently, there's no way to
ensure they support exactly the same capabilities. The kernel can
always lag. The only options are to allow the host kernel to nack
features, or to fall back to the userspace implementation.
It needs to be finer grained (qemu invoker telling qemu what the minimum
features are needed, and qemu telling the invoker what capabilties it
supports) but there's no way around it IMO.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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