Re: [PATCH v2 18/21] arm64: KVM: Introduce hyp_alternate_value helper

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Feb 02, 2016 at 01:42:08PM +0000, Marc Zyngier wrote:
> On 01/02/16 14:41, Christoffer Dall wrote:
> > On Mon, Jan 25, 2016 at 03:53:52PM +0000, Marc Zyngier wrote:
> >> We already have hyp_alternate_select() to define a function pointer
> >> that gets changed by a kernel feature or workaround.
> >>
> >> It would be useful to have a similar feature that resolves in a
> >> direct value, without requiring a function call. For this purpose,
> >> introduce hyp_alternate_value(), which returns one of two values
> >> depending on the state of the alternative.
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >> ---
> >>  arch/arm64/kvm/hyp/hyp.h | 11 +++++++++++
> >>  1 file changed, 11 insertions(+)
> >>
> >> diff --git a/arch/arm64/kvm/hyp/hyp.h b/arch/arm64/kvm/hyp/hyp.h
> >> index 44eaff7..dc75fdb 100644
> >> --- a/arch/arm64/kvm/hyp/hyp.h
> >> +++ b/arch/arm64/kvm/hyp/hyp.h
> >> @@ -144,6 +144,17 @@ typeof(orig) * __hyp_text fname(void)					\
> >>  	return val;							\
> >>  }
> >>  
> >> +#define hyp_alternate_value(fname, orig, alt, cond)			\
> >> +typeof(orig) __hyp_text fname(void)					\
> >> +{									\
> >> +	typeof(alt) val = orig;						\
> >> +	asm volatile(ALTERNATIVE("nop		\n",			\
> >> +				 "mov	%0, %1	\n",			\
> >> +				 cond)					\
> >> +		     : "+r" (val) : "r" ((typeof(orig))alt));		\
> >> +	return val;							\
> >> +}
> >> +
> >>  void __vgic_v2_save_state(struct kvm_vcpu *vcpu);
> >>  void __vgic_v2_restore_state(struct kvm_vcpu *vcpu);
> >>  
> >> -- 
> >> 2.1.4
> >>
> > I'm really not convinced that this is more readable than simply defining
> > a function where needed.  Perhaps the thing that needs a definition is
> > the "asm volatile(ALTERNATIVE(...))" part?  I also don't see why any of
> > this is specific to KVM or Hyp ?
> 
> I can easily factor out the whole asm volatile part. What I'm trying to
> avoid is an additional function call, but maybe we shouldn't need to
> worry about the overhead on page faults altogether?
> 
> I'll drop it for now, and we can reconsider it later.
> 
Sounds good to me.

Thanks,
-Christoffer