linux-kernel - Re: [PATCH RESEND 2/3] KVM: Add paravirt remote TLB flush

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Date:   Thu, 9 Nov 2017 19:01:43 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Eduardo Valentin <eduval@...zon.com>
Subject: Re: [PATCH RESEND 2/3] KVM: Add paravirt remote TLB flush

2017-11-09 18:48 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> On 09/11/2017 03:02, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>> 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@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>>  arch/x86/include/uapi/asm/kvm_para.h |  1 +
>>  arch/x86/kernel/kvm.c                | 29 +++++++++++++++++++++++++++++
>>  2 files changed, 30 insertions(+)
>>
>> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
>> index ff23ce9..189e354 100644
>> --- a/arch/x86/include/uapi/asm/kvm_para.h
>> +++ b/arch/x86/include/uapi/asm/kvm_para.h
>> @@ -52,6 +52,7 @@ 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 1b1b641..2e2f3ae 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 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_t flushmask;
>> +
>> +
>> +     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 | 1 <<
>> +                             KVM_VCPU_SHOULD_FLUSH))
>> +                                     cpumask_clear_cpu(cpu, &flushmask);
>> +             }
>> +     }
>> +
>> +     native_flush_tlb_others(&flushmask, info);
>> +}
>> +
>>  void __init kvm_guest_init(void)
>>  {
>>       int i;
>> @@ -484,6 +511,8 @@ void __init kvm_guest_init(void)
>>               pv_time_ops.steal_clock = kvm_steal_clock;
>>       }
>>
>> +     pv_mmu_ops.flush_tlb_others = kvm_flush_tlb_others;
>
> This needs to be keyed on a new CPUID feature bit.  Eduardo is also

Will do.

> adding a new "PV_DEDICATED" hint and you might disable PV TLB flush when
> PV_DEDICATED is set.

Why disable PV TLB flush for PV_DEDICATED(qspinlock)?

Regards,
Wanpeng Li