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

Catalin Marinas
2011-12-01T16:02:41Z

On Thu, Dec 01, 2011 at 03:42:19PM +0000, Arnd Bergmann wrote:
> On Thursday 01 December 2011, Catalin Marinas wrote:
> > 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.
>
> How do you deal with signed integer arguments passed into SVC or HVC from
> a caller? If I understand the architecture correctly, the upper
> halves of the argument register end up zero-padded, while the callee
> expects sign-extension.

If you treat it as an "int" (32-bit) and function prototype defined
accordingly, then the generated code only accesses it as a W (rather
than X) register and the top 32-bit part is ignored (no need for
sign-extension). If it is defined as a "long" in the 32-bit world, then
it indeed needs explicit conversion given the different sizes for long
(for example sys_lseek, the second argument is a 'long' and we do
explicit sign extension in the wrapper).

--
Catalin