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

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Dec 01, 2011 at 10:26:37AM +0000, 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.

Given the way register banking is done on AArch64, issuing an HVC on a
32-bit guest OS doesn't require translation on a 64-bit hypervisor. We
have a similar implementation at the SVC level (for 32-bit user apps on
a 64-bit kernel), the only modification was where a 32-bit SVC takes a
64-bit parameter in two separate 32-bit registers, so packing needs to
be done in a syscall wrapper.

I'm not closely involved with any of the Xen or KVM work but I would
vote for using HVC than a hypercall page.

-- 
Catalin