Re: [PATCH v2 2/4] KVM: Add paravirt remote TLB flush

[Wanpeng Li]

2017-11-10 16:33 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>:
> 2017-11-10 16:24 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>> On 10/11/2017 08:04, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>>
>>> Remote flushing api's does a busy wait which is fine in bare-metal
>>> scenario. But with-in the guest, the vcpus might have been pre-empted
>>> or blocked. In this scenario, the initator vcpu would end up
>>> busy-waiting for a long amount of time.
>>>
>>> This patch set implements para-virt flush tlbs making sure that it
>>> does not wait for vcpus that are sleeping. And all the sleeping vcpus
>>> flush the tlb on guest enter.
>>>
>>> The best result is achieved when we're overcommiting the host by running
>>> multiple vCPUs on each pCPU. In this case PV tlb flush avoids touching
>>> vCPUs which are not scheduled and avoid the wait on the main CPU.
>>>
>>> Test on a Haswell i7 desktop 4 cores (2HT), so 8 pCPUs, running ebizzy in
>>> one linux guest.
>>>
>>> ebizzy -M
>>>               vanilla    optimized     boost
>>>  8 vCPUs       10152       10083       -0.68%
>>> 16 vCPUs        1224        4866       297.5%
>>> 24 vCPUs        1109        3871       249%
>>> 32 vCPUs        1025        3375       229.3%
>>>
>>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>> ---
>>>  Documentation/virtual/kvm/cpuid.txt  |  4 ++++
>>>  arch/x86/include/uapi/asm/kvm_para.h |  2 ++
>>>  arch/x86/kernel/kvm.c                | 31 +++++++++++++++++++++++++++++++
>>>  3 files changed, 37 insertions(+)
>>>
>>> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
>>> index 117066a..9693fcc 100644
>>> --- a/Documentation/virtual/kvm/cpuid.txt
>>> +++ b/Documentation/virtual/kvm/cpuid.txt
>>> @@ -60,6 +60,10 @@ KVM_FEATURE_PV_DEDICATED           ||     8 || guest checks this feature bit
>>>                                     ||       || mizations such as usage of
>>>                                     ||       || qspinlocks.
>>>  ------------------------------------------------------------------------------
>>> +KVM_FEATURE_PV_TLB_FLUSH           ||     9 || guest checks this feature bit
>>> +                                   ||       || before enabling paravirtualized
>>> +                                   ||       || tlb flush.
>>> +------------------------------------------------------------------------------
>>>  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
>>>                                     ||       || per-cpu warps are expected in
>>>                                     ||       || kvmclock.
>>> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
>>> index 9ead1ed..a028479 100644
>>> --- a/arch/x86/include/uapi/asm/kvm_para.h
>>> +++ b/arch/x86/include/uapi/asm/kvm_para.h
>>> @@ -25,6 +25,7 @@
>>>  #define KVM_FEATURE_PV_EOI           6
>>>  #define KVM_FEATURE_PV_UNHALT                7
>>>  #define KVM_FEATURE_PV_DEDICATED     8
>>> +#define KVM_FEATURE_PV_TLB_FLUSH     9
>>>
>>>  /* The last 8 bits are used to indicate how to interpret the flags field
>>>   * in pvclock structure. If no bits are set, all flags are ignored.
>>> @@ -53,6 +54,7 @@ struct kvm_steal_time {
>>>
>>>  #define KVM_VCPU_NOT_PREEMPTED      (0 << 0)
>>>  #define KVM_VCPU_PREEMPTED          (1 << 0)
>>> +#define KVM_VCPU_SHOULD_FLUSH       (1 << 1)
>>>
>>>  #define KVM_CLOCK_PAIRING_WALLCLOCK 0
>>>  struct kvm_clock_pairing {
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index 66ed3bc..50f4b6a 100644
>>> --- a/arch/x86/kernel/kvm.c
>>> +++ b/arch/x86/kernel/kvm.c
>>> @@ -465,6 +465,33 @@ static void __init kvm_apf_trap_init(void)
>>>       update_intr_gate(X86_TRAP_PF, async_page_fault);
>>>  }
>>>
>>> +static cpumask_t flushmask;
>>
>> Hi Wanpeng,
>>
>> are you going to send v3 with a percpu variable?
>
> Yeah, I just complete v3 according to Peterz's comments in another
> guy's thread, I will send out them after completing the testing.

This is how it looks it. https://pastebin.com/raw/L2vqu4cZ

Regards,
Wanpeng Li

>
> Regards,
> Wanpeng Li
>
>>
>> Paolo
>>
>>> +static void kvm_flush_tlb_others(const struct cpumask *cpumask,
>>> +                     const struct flush_tlb_info *info)
>>> +{
>>> +     u8 state;
>>> +     int cpu;
>>> +     struct kvm_steal_time *src;
>>> +
>>> +     cpumask_copy(&flushmask, cpumask);
>>> +     /*
>>> +      * We have to call flush only on online vCPUs. And
>>> +      * queue flush_on_enter for pre-empted vCPUs
>>> +      */
>>> +     for_each_cpu(cpu, cpumask) {
>>> +             src = &per_cpu(steal_time, cpu);
>>> +             state = src->preempted;
>>> +             if ((state & KVM_VCPU_PREEMPTED)) {
>>> +                     if (cmpxchg(&src->preempted, state, state |
>>> +                             KVM_VCPU_SHOULD_FLUSH) == state)
>>> +                             __cpumask_clear_cpu(cpu, &flushmask);
>>> +             }
>>> +     }
>>> +
>>> +     native_flush_tlb_others(&flushmask, info);
>>> +}
>>> +
>>>  void __init kvm_guest_init(void)
>>>  {
>>>       int i;
>>> @@ -484,6 +511,10 @@ void __init kvm_guest_init(void)
>>>               pv_time_ops.steal_clock = kvm_steal_clock;
>>>       }
>>>
>>> +     if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
>>> +             !kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED))
>>> +             pv_mmu_ops.flush_tlb_others = kvm_flush_tlb_others;
>>> +
>>>       if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>>>               apic_set_eoi_write(kvm_guest_apic_eoi_write);
>>>
>>>
>>