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SubjectRe: [PATCH 5/5] KVM: arm64: Support the vcpu preemption check
On Tue, Dec 17, 2019 at 01:55:49PM +0000, yezengruan@huawei.com wrote:
> From: Zengruan Ye <yezengruan@huawei.com>
>
> 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.
>
> unix benchmark result:
> host: kernel 5.5.0-rc1, HiSilicon Kunpeng920, 8 cpus
> guest: kernel 5.5.0-rc1, 16 vcpus
>
> test-case | after-patch | before-patch
> ----------------------------------------+-------------------+------------------
> Dhrystone 2 using register variables | 334600751.0 lps | 335319028.3 lps
> Double-Precision Whetstone | 32856.1 MWIPS | 32849.6 MWIPS
> Execl Throughput | 3662.1 lps | 2718.0 lps
> File Copy 1024 bufsize 2000 maxblocks | 432906.4 KBps | 158011.8 KBps
> File Copy 256 bufsize 500 maxblocks | 116023.0 KBps | 37664.0 KBps
> File Copy 4096 bufsize 8000 maxblocks | 1432769.8 KBps | 441108.8 KBps
> Pipe Throughput | 6405029.6 lps | 6021457.6 lps
> Pipe-based Context Switching | 185872.7 lps | 184255.3 lps
> Process Creation | 4025.7 lps | 3706.6 lps
> Shell Scripts (1 concurrent) | 6745.6 lpm | 6436.1 lpm
> Shell Scripts (8 concurrent) | 998.7 lpm | 931.1 lpm
> System Call Overhead | 3913363.1 lps | 3883287.8 lps
> ----------------------------------------+-------------------+------------------
> System Benchmarks Index Score | 1835.1 | 1327.6
>
> Signed-off-by: Zengruan Ye <yezengruan@huawei.com>
> ---
> arch/arm64/include/asm/paravirt.h | 3 +
> arch/arm64/kernel/paravirt.c | 91 +++++++++++++++++++++++++++++++
> arch/arm64/kernel/setup.c | 2 +
> include/linux/cpuhotplug.h | 1 +
> 4 files changed, 97 insertions(+)
>
> diff --git a/arch/arm64/include/asm/paravirt.h b/arch/arm64/include/asm/paravirt.h
> index 7b1c81b544bb..a2cd0183bbef 100644
> --- a/arch/arm64/include/asm/paravirt.h
> +++ b/arch/arm64/include/asm/paravirt.h
> @@ -29,6 +29,8 @@ static inline u64 paravirt_steal_clock(int cpu)
>
> int __init pv_time_init(void);
>
> +int __init kvm_guest_init(void);
> +

This is a *very* generic name - I suggest something like pv_lock_init()
so it's clear what the function actually does.

> __visible bool __native_vcpu_is_preempted(int cpu);
>
> static inline bool pv_vcpu_is_preempted(int cpu)
> @@ -39,6 +41,7 @@ static inline bool pv_vcpu_is_preempted(int cpu)
> #else
>
> #define pv_time_init() do {} while (0)
> +#define kvm_guest_init() do {} while (0)
>
> #endif // CONFIG_PARAVIRT
>
> diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
> index d8f1ba8c22ce..a86dead40473 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/smp_plat.h>
> +#include <asm/pvlock-abi.h>
>
> struct static_key paravirt_steal_enabled;
> struct static_key paravirt_steal_rq_enabled;
> @@ -158,3 +159,93 @@ int __init pv_time_init(void)
>
> return 0;
> }
> +
> +DEFINE_PER_CPU(struct pvlock_vcpu_state, pvlock_vcpu_region) __aligned(64);
> +EXPORT_PER_CPU_SYMBOL(pvlock_vcpu_region);
> +
> +static int pvlock_vcpu_state_dying_cpu(unsigned int cpu)
> +{
> + struct pvlock_vcpu_state *reg;
> +
> + reg = this_cpu_ptr(&pvlock_vcpu_region);
> + if (!reg)
> + return -EFAULT;
> +
> + memset(reg, 0, sizeof(*reg));

I might be missing something obvious here - but I don't see the point of
this. The hypervisor might immediately overwrite the structure again.
Indeed you should conside a mechanism for the guest to "unregister" the
region - otherwise you will face issues with the likes of kexec.

For pv_time the memory is allocated by the hypervisor not the guest to
avoid lifetime issues about kexec.

> +
> + return 0;
> +}
> +
> +static int init_pvlock_vcpu_state(unsigned int cpu)
> +{
> + struct pvlock_vcpu_state *reg;
> + struct arm_smccc_res res;
> +
> + reg = this_cpu_ptr(&pvlock_vcpu_region);
> + if (!reg)
> + return -EFAULT;
> +
> + /* Pass the memory address to host via hypercall */
> + arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_LOCK_PREEMPTED,
> + virt_to_phys(reg), &res);
> +
> + return 0;
> +}
> +
> +static bool kvm_vcpu_is_preempted(int cpu)
> +{
> + struct pvlock_vcpu_state *reg = &per_cpu(pvlock_vcpu_region, cpu);
> +
> + if (reg)
> + return !!(reg->preempted & 1);
> +
> + return false;
> +}
> +
> +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)
> + return ret;
> +
> + pv_ops.lock.vcpu_is_preempted = kvm_vcpu_is_preempted;
> +
> + pr_info("using PV-lock preempted\n");
> +
> + 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 (psci_ops.smccc_version < SMCCC_VERSION_1_1)
> + return false;
> +
> + arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
> + ARM_SMCCC_HV_PV_LOCK_FEATURES, &res);
> +
> + if (res.a0 != SMCCC_RET_SUCCESS)
> + return false;
> +
> + return true;
> +}
> +
> +int __init kvm_guest_init(void)
> +{
> + if (is_hyp_mode_available())
> + return 0;
> +
> + if (!has_kvm_pvlock())
> + return 0;
> +
> + kvm_arm_init_pvlock();
Consider reporting errors from kvm_arm_init_pvlock()? At the moment
it's impossible to tell the difference between pvlock not being
supported and something failing in the setup.

Steve

> +
> + return 0;
> +}
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 56f664561754..64c4d515ba2d 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -341,6 +341,8 @@ void __init setup_arch(char **cmdline_p)
> smp_init_cpus();
> smp_build_mpidr_hash();
>
> + kvm_guest_init();
> +
> /* Init percpu seeds for random tags after cpus are set up. */
> kasan_init_tags();
>
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index e51ee772b9f5..f72ff95ab63a 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -138,6 +138,7 @@ enum cpuhp_state {
> CPUHP_AP_DUMMY_TIMER_STARTING,
> CPUHP_AP_ARM_XEN_STARTING,
> CPUHP_AP_ARM_KVMPV_STARTING,
> + CPUHP_AP_ARM_KVM_PVLOCK_STARTING,
> CPUHP_AP_ARM_CORESIGHT_STARTING,
> CPUHP_AP_ARM64_ISNDEP_STARTING,
> CPUHP_AP_SMPCFD_DYING,
> --
> 2.19.1
>
>

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