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

Stefano Stabellini
2011-12-01T15:12:06Z

On Thu, 1 Dec 2011, Ian Campbell wrote:
> 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.

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.