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 <[email protected]> > >> --- > >> 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
