Re: [External] Re: [RFC 5/6] KVM: arm64: Support the VCPU preemption check

[Usama Arif <usama.arif@xxxxxxxxxxxxx>]

On 03/11/2022 13:25, Steven Price wrote:
> On 02/11/2022 16:13, Usama Arif wrote:
> > Support the vcpu_is_preempted() functionality under KVM/arm64. This will
> > enhance lock performance on overcommitted hosts (more runnable VCPUs
> > than physical CPUs in the system) as doing busy waits for preempted
> > VCPUs will hurt system performance far worse than early yielding.
> >
> > Signed-off-by: Zengruan Ye <yezengruan@xxxxxxxxxx>
> > Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
> > ---
> >   arch/arm64/include/asm/paravirt.h |   2 +
> >   arch/arm64/include/asm/spinlock.h |  16 +++-
> >   arch/arm64/kernel/paravirt.c      | 126 ++++++++++++++++++++++++++++++
> >   arch/arm64/kernel/setup.c         |   3 +
> >   include/linux/cpuhotplug.h        |   1 +
> >   5 files changed, 147 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/paravirt.h b/arch/arm64/include/asm/paravirt.h
> > index 9aa193e0e8f2..4ccb4356c56b 100644
> > --- a/arch/arm64/include/asm/paravirt.h
> > +++ b/arch/arm64/include/asm/paravirt.h
> > @@ -19,10 +19,12 @@ static inline u64 paravirt_steal_clock(int cpu)
> >   }
> >   
> >   int __init pv_time_init(void);
> > +int __init pv_lock_init(void);
> >   
> >   #else
> >   
> >   #define pv_time_init() do {} while (0)
> > +#define pv_lock_init() do {} while (0)
> >   
> >   #endif // CONFIG_PARAVIRT
> >   
> > diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
> > index 0525c0b089ed..7023efa4de96 100644
> > --- a/arch/arm64/include/asm/spinlock.h
> > +++ b/arch/arm64/include/asm/spinlock.h
> > @@ -10,7 +10,20 @@
> >   
> >   /* See include/linux/spinlock.h */
> >   #define smp_mb__after_spinlock()	smp_mb()
> > +#define vcpu_is_preempted vcpu_is_preempted
> > +
> > +#ifdef CONFIG_PARAVIRT
> > +#include <linux/static_call_types.h>
> > +
> > +bool dummy_vcpu_is_preempted(int cpu);
> >   
> > +DECLARE_STATIC_CALL(pv_vcpu_is_preempted, dummy_vcpu_is_preempted);
> > +static inline bool vcpu_is_preempted(int cpu)
> > +{
> > +	return static_call(pv_vcpu_is_preempted)(cpu);
> > +}
> > +
> > +#else
> >   /*
> >    * Changing this will break osq_lock() thanks to the call inside
> >    * smp_cond_load_relaxed().
> > @@ -18,10 +31,11 @@
> >    * See:
> >    * https://lore.kernel.org/lkml/20200110100612.GC2827@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> >    */
> > -#define vcpu_is_preempted vcpu_is_preempted
> >   static inline bool vcpu_is_preempted(int cpu)
> >   {
> >   	return false;
> >   }
> >   
> > +#endif /* CONFIG_PARAVIRT */
> > +
> >   #endif /* __ASM_SPINLOCK_H */
> > diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
> > index 57c7c211f8c7..45bcca87bed7 100644
> > --- a/arch/arm64/kernel/paravirt.c
> > +++ b/arch/arm64/kernel/paravirt.c
> > @@ -22,6 +22,7 @@
> >   
> >   #include <asm/paravirt.h>
> >   #include <asm/pvclock-abi.h>
> > +#include <asm/pvlock-abi.h>
> >   #include <asm/smp_plat.h>
> >   
> >   struct static_key paravirt_steal_enabled;
> > @@ -38,7 +39,12 @@ struct pv_time_stolen_time_region {
> >   	struct pvclock_vcpu_stolen_time __rcu *kaddr;
> >   };
> >   
> > +struct pv_lock_state_region {
> > +	struct pvlock_vcpu_state __rcu *kaddr;
> > +};
> > +
> >   static DEFINE_PER_CPU(struct pv_time_stolen_time_region, stolen_time_region);
> > +static DEFINE_PER_CPU(struct pv_lock_state_region, lock_state_region);
> >   
> >   static bool steal_acc = true;
> >   static int __init parse_no_stealacc(char *arg)
> > @@ -178,3 +184,123 @@ int __init pv_time_init(void)
> >   
> >   	return 0;
> >   }
> > +
> > +static bool native_vcpu_is_preempted(int cpu)
> > +{
> > +	return false;
> > +}
> > +
> > +DEFINE_STATIC_CALL(pv_vcpu_is_preempted, native_vcpu_is_preempted);
> > +
> > +static bool para_vcpu_is_preempted(int cpu)
> > +{
> > +	struct pv_lock_state_region *reg;
> > +	__le64 preempted_le;
> > +
> > +	reg = per_cpu_ptr(&lock_state_region, cpu);
> > +	if (!reg->kaddr) {
> > +		pr_warn_once("PV lock enabled but not configured for cpu %d\n",
> > +			     cpu);
> > +		return false;
> > +	}
> > +
> > +	preempted_le = le64_to_cpu(READ_ONCE(reg->kaddr->preempted));
> > +
> > +	return !!(preempted_le);
> > +}
> > +
> > +static int pvlock_vcpu_state_dying_cpu(unsigned int cpu)
> > +{
> > +	struct pv_lock_state_region *reg;
> > +
> > +	reg = this_cpu_ptr(&lock_state_region);
> > +	if (!reg->kaddr)
> > +		return 0;
> > +
> > +	memunmap(reg->kaddr);
> > +	memset(reg, 0, sizeof(*reg));
> > +
> > +	return 0;
> > +}
> > +
> > +static int init_pvlock_vcpu_state(unsigned int cpu)
> > +{
> > +	struct pv_lock_state_region *reg;
> > +	struct arm_smccc_res res;
> > +
> > +	reg = this_cpu_ptr(&lock_state_region);
> > +
> > +	arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_LOCK_PREEMPTED, &res);
> > +
> > +	if (res.a0 == SMCCC_RET_NOT_SUPPORTED) {
> > +		pr_warn("Failed to init PV lock data structure\n");
> > +		return -EINVAL;
> > +	}
> > +
> > +	reg->kaddr = memremap(res.a0,
> > +			      sizeof(struct pvlock_vcpu_state),
> > +			      MEMREMAP_WB);
> > +
> > +	if (!reg->kaddr) {
> > +		pr_warn("Failed to map PV lock data structure\n");
> > +		return -ENOMEM;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static int kvm_arm_init_pvlock(void)
> > +{
> > +	int ret;
> > +
> > +	ret = cpuhp_setup_state(CPUHP_AP_ARM_KVM_PVLOCK_STARTING,
> > +				"hypervisor/arm/pvlock:starting",
> > +				init_pvlock_vcpu_state,
> > +				pvlock_vcpu_state_dying_cpu);
> > +	if (ret < 0) {
> > +		pr_warn("PV-lock init failed\n");
> > +		return ret;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static bool has_kvm_pvlock(void)
> > +{
> > +	struct arm_smccc_res res;
> > +
> > +	/* To detect the presence of PV lock support we require SMCCC 1.1+ */
> > +	if (arm_smccc_1_1_get_conduit() == SMCCC_CONDUIT_NONE)
> > +		return false;
>
> This is unnecessary as arm_smccc_1_1_invoke() will return failure if
> there's no conduit (or pre-SMCCC 1.1). I suspect this was a copy/paste
> from has_pv_steal_clock() which also has the unnecessary check (patch
> welcome ;) ).


Thanks for the review!

I have sent a seperate patch to remove it from pv_steal_clock: 
https://lore.kernel.org/all/20221104061659.4116508-1-usama.arif@xxxxxxxxxxxxx/. 


I have removed it from pv_lock as well in the v2 patchset: 
https://lore.kernel.org/all/20221104062105.4119003-1-usama.arif@xxxxxxxxxxxxx/.
> > +
> > +	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
> > +			     ARM_SMCCC_HV_PV_LOCK_FEATURES, &res);
>
> Since this is a 'OWNER_VENDOR_HYP' call this should really be preceded
> by a check that we're running under the expected hypervisor. See e.g.
> kvm_init_hyp_services().
>
> Of course for KVM we already have a (different) discovery mechanism and
> this could be included as a ARM_SMCCC_KVM_FUNC_xxx feature. This
> has_kvm_pvlock() function would then simply be:
>
> static bool has_kvm_pvlock(void)
> {
> 	return kvm_arm_hyp_service_available(ARM_SMCC_KVM_FUNC_PVLOCK);
> }

Thanks, I have simplified has_kvm_pvlock to above. I also changed the 
code in the v2 revision, so that its just 1 hypercall, 
ARM_SMCCC_VENDOR_HYP_KVM_PV_LOCK_FUNC_ID. This is similar to how it is 
done in ptp_kvm vendor call.

Usama

> Steve
>
> > +
> > +	if (res.a0 != SMCCC_RET_SUCCESS)
> > +		return false;
> > +
> > +	arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_LOCK_FEATURES,
> > +			     ARM_SMCCC_HV_PV_LOCK_PREEMPTED, &res);
> > +
> > +	return (res.a0 == SMCCC_RET_SUCCESS);
> > +}
> > +
> > +int __init pv_lock_init(void)
> > +{
> > +	int ret;
> > +
> > +	if (is_hyp_mode_available())
> > +		return 0;
> > +
> > +	if (!has_kvm_pvlock())
> > +		return 0;
> > +
> > +	ret = kvm_arm_init_pvlock();
> > +	if (ret)
> > +		return ret;
> > +
> > +	static_call_update(pv_vcpu_is_preempted, para_vcpu_is_preempted);
> > +	pr_info("using PV-lock preempted\n");
> > +
> > +	return 0;
> > +}
> > \ No newline at end of file
> > diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> > index fea3223704b6..05ca07ac5800 100644
> > --- a/arch/arm64/kernel/setup.c
> > +++ b/arch/arm64/kernel/setup.c
> > @@ -42,6 +42,7 @@
> >   #include <asm/cpu_ops.h>
> >   #include <asm/kasan.h>
> >   #include <asm/numa.h>
> > +#include <asm/paravirt.h>
> >   #include <asm/sections.h>
> >   #include <asm/setup.h>
> >   #include <asm/smp_plat.h>
> > @@ -360,6 +361,8 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
> >   	smp_init_cpus();
> >   	smp_build_mpidr_hash();
> >   
> > +	pv_lock_init();
> > +
> >   	/* Init percpu seeds for random tags after cpus are set up. */
> >   	kasan_init_sw_tags();
> >   
> > diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> > index f61447913db9..c0ee11855c73 100644
> > --- a/include/linux/cpuhotplug.h
> > +++ b/include/linux/cpuhotplug.h
> > @@ -192,6 +192,7 @@ enum cpuhp_state {
> >   	/* Must be the last timer callback */
> >   	CPUHP_AP_DUMMY_TIMER_STARTING,
> >   	CPUHP_AP_ARM_XEN_STARTING,
> > +	CPUHP_AP_ARM_KVM_PVLOCK_STARTING,
> >   	CPUHP_AP_ARM_CORESIGHT_STARTING,
> >   	CPUHP_AP_ARM_CORESIGHT_CTI_STARTING,
> >   	CPUHP_AP_ARM64_ISNDEP_STARTING,