Re: [Xen-devel] [PATCH v2 for 4.5] arm32: fix build after 063188f4b3

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Stefano,

On 10/13/2014 05:06 PM, Stefano Stabellini wrote:
>> +int do_smc(register_t function_id, ...);
> 
> I am not sure whether this is safe: the smc calling convention on arm64
> doesn't promise to save the x0-x17 registers. The smc calling convention
> on arm32 only promises to save r4-r15. That means that after issuing an
> smc call with just two arguments, you could still find r3 to be changed
> afterwards.
> I think you'll have to manually save/restore all the registers outside
> the safety guarantees of the smc protocol.

The calling convention specifies:
        - r0-r3 as argument/scratch register on ARM32
        - x0-x17 as argument/temporary register on ARM64

So we don't need to save/restore them before/after calling the SMC function.

Regards,

-- 
Julien Grall

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