[PATCH 04/13] arm64: Add new hcall HVC_CALL_FUNC

[Geoff Levand]

Hi Mark,

On Mon, 2014-09-15 at 19:11 +0100, Mark Rutland wrote:
> On Tue, Sep 09, 2014 at 11:49:04PM +0100, Geoff Levand wrote:
> > Add the new hcall HVC_CALL_FUNC that allows execution of a function at EL2.
> > During CPU reset the CPU must be brought to the exception level it had on
> > entry to the kernel.  The HVC_CALL_FUNC hcall will provide the mechanism
> > needed for this exception level switch.
> > 
> > Signed-off-by: Geoff Levand <geoff at infradead.org>
> > ---
> >  arch/arm64/include/asm/virt.h | 11 +++++++++++
> >  arch/arm64/kernel/hyp-stub.S  | 10 ++++++++++
> >  2 files changed, 21 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> > index 894fe53..b217fbc 100644
> > --- a/arch/arm64/include/asm/virt.h
> > +++ b/arch/arm64/include/asm/virt.h
> > @@ -41,6 +41,17 @@
> >  
> >  #define HVC_KVM_CALL_HYP 3
> >  
> > +/*
> > + * HVC_CALL_FUNC - Execute a function at EL2.
> > + *
> > + * @x0: Physical address of the funtion to be executed.
> > + * @x1: Passed as the first argument to @fn.
> > + * @x2: Passed as the second argument to @fn.
> > + * @x3: Passed as the third argument to @fn.
> > + */
> > +
> > +#define HVC_CALL_FUNC 4
> > +
> 
> Can't we use the HVC_KVM_CALL_HYP for this as well? I thought we already
> added the code to the stub to do that in the last patch.

The last patch (arm64: Convert hcalls to use ISS field) only added
function call support to the kvm vector.

> Is there a difference between the two that I'm missing?

The kvm and stub el1_sync vectors are different in that the kvm vector
expects a stack to be set up whereas the stub vector does not.  We have
plenty of hcall numbers, so I thought it better to have two different
numbers so improper usage would be easier to detect.  

> >  #ifndef __ASSEMBLY__
> >  
> >  /*
> > diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> > index 9ab5f70..a21cf51 100644
> > --- a/arch/arm64/kernel/hyp-stub.S
> > +++ b/arch/arm64/kernel/hyp-stub.S
> > @@ -75,7 +75,17 @@ el1_sync:
> >  1:	cmp	x10, #HVC_SET_VECTORS
> >  	b.ne	1f
> >  	msr	vbar_el2, x0
> > +	b	2f
> >  
> > +1:	cmp	x10, #HVC_CALL_FUNC
> > +	b.ne    1f
> > +	mov	x29, lr
> 
> What's the contract for functions we call through the stub?

My thinking was that this is just a mechanism to do the call.
The caller and callee would need to set the conditions of the
call.  The only restriction is that the called function would
need to preserve the register that lr is saved in (x29 here,
but x18 in the current implementation).

-Geoff