Messages in this thread | | | Subject | Re: [PATCH 02/24] xen/arm: hypercalls | From | Ian Campbell <> | Date | Fri, 27 Jul 2012 10:19:24 +0100 |
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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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