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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
