Re: [Embeddedxen-devel] [Xen-devel] [ANNOUNCE] Xen port to Cortex-A15 / ARMv7 with virt extensions

Ian Campbell
2011-12-01T10:26:37Z

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

>
> > 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.

Ian.