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

From: [email-masked] (Christoffer Dall)
Date: Thu, 15 Sep 2016 11:03:38 +0200

On Wed, Sep 14, 2016 at 04:20:00PM +0100, Vladimir Murzin wrote:
> On 13/09/16 10:22, Christoffer Dall wrote:
> > 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 <[email-masked]>
> >>>> ---
> >>>>  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!
> 
> To make it clear, should I respin with "enabled" into the info message
> and macros for static keys?
> 
No, I can fix up the info message and we can worry aboutt he macros
later.  Marc said he would be happy to do that :)

-Christoffer