Re: [PATCH v9 1/7] smccc: Add HVC call variant with result registers other than 0 thru 3

[Mark Rutland]

On Thu, Mar 25, 2021 at 04:55:51AM +0000, Michael Kelley wrote:
> From: Mark Rutland <mark.rutland@xxxxxxx> Sent: Wednesday, March 24, 2021 9:55 AM
> > For the benefit of others here, SMCCCv1.2 allows:
> > 
> > * SMC64/HVC64 to use all of x1-x17 for both parameters and return values
> > * SMC32/HVC32 to use all of r1-r7 for both parameters and return values
> > 
> > The rationale for this was to make it possible to pass a large number of
> > arguments in one call without the hypervisor/firmware needing to access
> > the memory of the caller.
> > 
> > My preference would be to add arm_smccc_1_2_{hvc,smc}() assembly
> > functions which read all the permitted argument registers from a struct,
> > and write all the permitted result registers to a struct, leaving it to
> > callers to set those up and decompose them.
> > 
> > That way we only have to write one implementation that all callers can
> > use, which'll be far easier to maintain. I suspect that in general the
> > cost of temporarily bouncing the values through memory will be dominated
> > by whatever the hypervisor/firmware is going to do, and if it's not we
> > can optimize that away in future.
> 
> Thanks for the feedback, and I'm working on implementing this approach.
> But I've hit a snag in that gcc limits the "asm" statement to 30 arguments,
> which gives us 15 registers as parameters and 15 registers as return
> values, instead of the 18 each allowed by SMCCC v1.2.  I will continue
> with the 15 register limit for now, unless someone knows a way to exceed
> that.  The alternative would be to go to pure assembly language.

I realise in retrospect this is not clear, but when I said "assembly
functions" I had meant raw assembly functions rather than inline
assembly.

We already have __arm_smccc_smc and __arm_smccc_hvc assembly functions
in arch/{arm,arm64}/kernel/smccc-call.S, and I'd expected we'd add the
full fat SMCCCv1.2 variants there.

Thanks,
Mark.