On Thu, 1 Dec 2011, Ian Campbell wrote:

On Wed, 2011-11-30 at 18:32 +0000, Stefano Stabellini wrote:

quoted

On Wed, 30 Nov 2011, Arnd Bergmann wrote:

quoted

KVM and Xen at least both fall into the single-return-value category,
so we should be able to agree on a calling conventions. KVM does not
have an hcall API on ARM yet, and I see no reason not to use the
same implementation that you have in the Xen guest.

Stefano, can you split out the generic parts of your asm/xen/hypercall.h
file into a common asm/hypercall.h and submit it for review to the
arm kernel list?

Sure, I can do that.
Usually the hypercall calling convention is very hypervisor specific,
but if it turns out that we have the same requirements I happy to design
a common interface.

I expect the only real decision to be made is hypercall page vs. raw hvc
instruction.

The page was useful on x86 where there is a variety of instructions
which could be used (at least for PV there was systenter/syscall/int, I
think vmcall instruction differs between AMD and Intel also) and gives
some additional flexibility. It's hard to predict but I don't think I'd
expect that to be necessary on ARM.

Another reason for having a hypercall page instead of a raw instruction
might be wanting to support 32 bit guests (from ~today) on a 64 bit
hypervisor in the future and perhaps needing to do some shimming/arg
translation. It would be better to aim for having the interface just be
32/64 agnostic but mistakes do happen.

I always like to keep things as simple as possible, so I am in favor of
using the raw hvc instruction.
Besides with the bulk of mmu hypercalls gone, it should not be difficult
to design a 32/64 bit agnostic interface.