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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 13 Oct 2014, Julien Grall wrote:
> 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.

I think you are right.

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