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SubjectRe: [PATCH 5/5] KVM: arm64: Support the vcpu preemption check
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Hi Steve,

On 2019/12/17 22:40, Steven Price wrote:
> 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.


Thanks for pointing it out to me! I'll update the memory allocation
mechanism of the PV lock structure 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.

Good point, I'll update the code.

>
> 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
>>
>>
>
> .
>

Thanks,

Zengruan

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