Re: [PATCH 09/14] KVM: PPC: Add generic single register ioctls

Marcelo Tosatti
2011-11-10T17:35:05Z

On Thu, Nov 10, 2011 at 05:49:42PM +0100, Alexander Graf wrote:
> >> Documentation/virtual/kvm/api.txt | 47 ++++++++++++++++++++++++++++++++++
> >> arch/powerpc/kvm/powerpc.c | 51 +++++++++++++++++++++++++++++++++++++
> >> include/linux/kvm.h | 32 +++++++++++++++++++++++
> >> 3 files changed, 130 insertions(+), 0 deletions(-)
> >I don't see the benefit of this generalization, the current structure where
> >context information is hardcoded in the data transmitted works well.
>
> Well, unfortunately it doesn't work quite as well for us because we
> are a much more evolving platform. Also, there are a lot of edges
> and corners of the architecture that simply aren't implemented in
> KVM as of now. I want to have something extensible enough so we
> don't break the ABI along the way.

You still have to agree on format between userspace and kernel, right?
If either party fails to conform to that, you're doomed.

The problem with two interfaces is potential ambiguity: is
register X implemented through KVM_GET_ONE_REG and also through
KVM_GET_XYZ_REGISTER_SET ? If its accessible by two interfaces, what is
the register writeback order? Is there a plan to convert, etc.

If you agree these concerns are valid, perhaps this interface can be PPC
specific.