Re: [Xen-devel] [PATCH 02/24] xen/arm: hypercalls

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2012-07-26 at 20:19 +0100, Christopher Covington wrote:
> Hi Stefano,
> 
> On 07/26/2012 11:33 AM, Stefano Stabellini wrote:
> > Use r12 to pass the hypercall number to the hypervisor.
> > 
> > We need a register to pass the hypercall number because we might not
> > know it at compile time and HVC only takes an immediate argument.
> 
> You're not going to JIT assemble the appropriate HVC instruction? Darn.

;-)

> How many call numbers are there, though? 8?

The maximum currently defined hypercall number is 55, although there are
some small gaps so there's actually more like 45 in total.

>  It seems like it'd be
> reasonable to take the approach that seems to be favored for MRC/MCR
> instructions, using a function containing switch statement that chooses
> between several inline assembly instructions based off an enum passed to
> the function. See for example arch_timer_reg_read in
> arch/arm/kernel/arch_timer.c.

I don't think it is feasible with this number of hypercalls, even
accepting that in many cases the number will be a constant so gcc can
likely optimise almost all of it away.

Is there something wrong with the r12 based approach?

Ian.


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