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Message-ID: <76f59579-b701-a243-2a50-72a1401d3a65@arm.com>
Date: Thu, 3 Nov 2022 13:25:27 +0000
From: Steven Price <steven.price@....com>
To: Usama Arif <usama.arif@...edance.com>,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
linux-doc@...r.kernel.org,
virtualization@...ts.linux-foundation.org, linux@...linux.org.uk,
yezengruan@...wei.com, catalin.marinas@....com, will@...nel.org,
maz@...nel.org, mark.rutland@....com
Cc: fam.zheng@...edance.com, liangma@...ngbit.com,
punit.agrawal@...edance.com
Subject: Re: [RFC 5/6] KVM: arm64: Support the VCPU preemption check
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@...wei.com>
> Signed-off-by: Usama Arif <usama.arif@...edance.com>
> ---
> 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@hirez.programming.kicks-ass.net
> */
> -#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 ;) ).
> +
> + 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);
}
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,
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