[PATCH 02/24] xen/arm: hypercalls
Ian.Campbell at citrix.com
Fri Jul 27 05:19:24 EDT 2012
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
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?
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