[PATCH v4 01/10] arm64: KVM: Use static keys for selecting the GIC backend

[Christoffer Dall]

On Tue, Sep 13, 2016 at 10:11:10AM +0100, Marc Zyngier wrote:
> On 13/09/16 09:20, Christoffer Dall wrote:
> > On Mon, Sep 12, 2016 at 03:49:15PM +0100, Vladimir Murzin wrote:
> >> Currently GIC backend is selected via alternative framework and this
> >> is fine. We are going to introduce vgic-v3 to 32-bit world and there
> >> we don't have patching framework in hand, so we can either check
> >> support for GICv3 every time we need to choose which backend to use or
> >> try to optimise it by using static keys. The later looks quite
> >> promising because we can share logic involved in selecting GIC backend
> >> between architectures if both uses static keys.
> >>
> >> This patch moves arm64 from alternative to static keys framework for
> >> selecting GIC backend. For that we embed static key into vgic_global
> >> and enable the key during vgic initialisation based on what has
> >> already been exposed by the host GIC driver.
> >>
> >> Signed-off-by: Vladimir Murzin <vladimir.murzin at arm.com>
> >> ---
> >>  arch/arm64/kvm/hyp/switch.c   |   21 +++++++++++----------
> >>  include/kvm/arm_vgic.h        |    4 ++++
> >>  virt/kvm/arm/vgic/vgic-init.c |    4 ++++
> >>  virt/kvm/arm/vgic/vgic.c      |    2 +-
> >>  4 files changed, 20 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> >> index 5a84b45..d5c4cc5 100644
> >> --- a/arch/arm64/kvm/hyp/switch.c
> >> +++ b/arch/arm64/kvm/hyp/switch.c
> >> @@ -16,6 +16,8 @@
> >>   */
> >>  
> >>  #include <linux/types.h>
> >> +#include <linux/jump_label.h>
> >> +
> >>  #include <asm/kvm_asm.h>
> >>  #include <asm/kvm_hyp.h>
> >>  
> >> @@ -126,17 +128,13 @@ static void __hyp_text __deactivate_vm(struct kvm_vcpu *vcpu)
> >>  	write_sysreg(0, vttbr_el2);
> >>  }
> >>  
> >> -static hyp_alternate_select(__vgic_call_save_state,
> >> -			    __vgic_v2_save_state, __vgic_v3_save_state,
> >> -			    ARM64_HAS_SYSREG_GIC_CPUIF);
> >> -
> >> -static hyp_alternate_select(__vgic_call_restore_state,
> >> -			    __vgic_v2_restore_state, __vgic_v3_restore_state,
> >> -			    ARM64_HAS_SYSREG_GIC_CPUIF);
> >> -
> >>  static void __hyp_text __vgic_save_state(struct kvm_vcpu *vcpu)
> >>  {
> >> -	__vgic_call_save_state()(vcpu);
> >> +	if (static_branch_unlikely(&kvm_vgic_global_state.gicv3_cpuif))
> > 
> > It's a bit weird that we use _unlikely for GICv3 (at least if/when GICv3
> > hardware becomes mainstream), but as we don't have another primitive for
> > the 'default disabled' case, I suppose that's the best we can do.
> 
> We could always revert the "likelihood" of that test once GICv3 has
> conquered the world. Or start patching the 32bit kernel like we do for
> 64bit...
> 
> > 
> >> +		__vgic_v3_save_state(vcpu);
> >> +	else
> >> +		__vgic_v2_save_state(vcpu);
> >> +
> >>  	write_sysreg(read_sysreg(hcr_el2) & ~HCR_INT_OVERRIDE, hcr_el2);
> >>  }
> >>  
> >> @@ -149,7 +147,10 @@ static void __hyp_text __vgic_restore_state(struct kvm_vcpu *vcpu)
> >>  	val |= vcpu->arch.irq_lines;
> >>  	write_sysreg(val, hcr_el2);
> >>  
> >> -	__vgic_call_restore_state()(vcpu);
> >> +	if (static_branch_unlikely(&kvm_vgic_global_state.gicv3_cpuif))
> >> +		__vgic_v3_restore_state(vcpu);
> >> +	else
> >> +		__vgic_v2_restore_state(vcpu);
> >>  }
> >>  
> >>  static bool __hyp_text __true_value(void)
> >> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
> >> index 19b698e..994665a 100644
> >> --- a/include/kvm/arm_vgic.h
> >> +++ b/include/kvm/arm_vgic.h
> >> @@ -23,6 +23,7 @@
> >>  #include <linux/types.h>
> >>  #include <kvm/iodev.h>
> >>  #include <linux/list.h>
> >> +#include <linux/jump_label.h>
> >>  
> >>  #define VGIC_V3_MAX_CPUS	255
> >>  #define VGIC_V2_MAX_CPUS	8
> >> @@ -63,6 +64,9 @@ struct vgic_global {
> >>  
> >>  	/* Only needed for the legacy KVM_CREATE_IRQCHIP */
> >>  	bool			can_emulate_gicv2;
> >> +
> >> +	/* GIC system register CPU interface */
> >> +	struct static_key_false gicv3_cpuif;
> > 
> > Documentation/static-keys.txt says that we are not supposed to use
> > struct static_key_false directly.  This will obviously work quite
> > nicely, but we could consider adding a pair of
> > DECLARE_STATIC_KEY_TRUE/FALSE macros that don't have the assignments,
> > but obviously this will need an ack from other maintainers.
> > 
> > Thoughts?
> 
> Grepping through the tree shows that we're not the only abusers of this
> (dynamic debug is far worse!). Happy to write the additional macros and
> submit them if nobody beats me to it.
> 
> > 
> > 
> >>  };
> >>  
> >>  extern struct vgic_global kvm_vgic_global_state;
> >> diff --git a/virt/kvm/arm/vgic/vgic-init.c b/virt/kvm/arm/vgic/vgic-init.c
> >> index 83777c1..14d6718 100644
> >> --- a/virt/kvm/arm/vgic/vgic-init.c
> >> +++ b/virt/kvm/arm/vgic/vgic-init.c
> >> @@ -405,6 +405,10 @@ int kvm_vgic_hyp_init(void)
> >>  		break;
> >>  	case GIC_V3:
> >>  		ret = vgic_v3_probe(gic_kvm_info);
> >> +		if (!ret) {
> >> +			static_branch_enable(&kvm_vgic_global_state.gicv3_cpuif);
> >> +			kvm_info("GIC system register CPU interface\n");
> > 
> > nit: add enabled to the info message?
> > 
> >> +		}
> >>  		break;
> >>  	default:
> >>  		ret = -ENODEV;
> >> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> >> index e83b7fe..8a529a7 100644
> >> --- a/virt/kvm/arm/vgic/vgic.c
> >> +++ b/virt/kvm/arm/vgic/vgic.c
> >> @@ -29,7 +29,7 @@
> >>  #define DEBUG_SPINLOCK_BUG_ON(p)
> >>  #endif
> >>  
> >> -struct vgic_global __section(.hyp.text) kvm_vgic_global_state;
> >> +struct vgic_global __section(.hyp.text) kvm_vgic_global_state = {.gicv3_cpuif = STATIC_KEY_FALSE_INIT,};
> >>  
> >>  /*
> >>   * Locking order is always:
> >> -- 
> >> 1.7.9.5
> >>
> > 
> > Overall this looks really nice, as long as we're clear on the static
> > keys stuff.
> 
> Indeed, we should get this sorted, though I'm not sure this should be a
> blocker for this code.
> 
Agreed, let's ship it!
-Christoffer