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

From: [email-masked] (Vladimir Murzin)
Date: Wed, 14 Sep 2016 16:20:00 +0100

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?

Cheers
Vladimir

> -Christoffer