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.