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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 27 Jul 2012, Christopher Covington wrote:
> On 07/27/2012 05:19 AM, Ian Campbell wrote:
> > 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.
> > 
> > ;-)
> > 

I admit having spent few hours thinking about how to implement a
self-modifying function able to change the ISS at run time. Fortunately
few hours later I was struck by common sense and I decided to follow a
different direction ;-)


> > 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?
> 
> Only that you're defining a custom interface for something that there is
> a potentially more standard interface for. I just wanted to double check
> that all the ways of using the potentially more standard interface had
> been explored and found to be unreasonable.

Yep, thanks for helping us reviewing the code.

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