linux-kernel - Re: [RFC PATCH 5/6] x86/mm/tlb: Flush remote and local TLBs concurrently

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Message-ID: <e9c0dc1f-799a-b6e3-8d41-58f0a6b693cd@redhat.com>
Date:   Mon, 27 May 2019 12:21:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>
Cc:     Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-hyperv@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [RFC PATCH 5/6] x86/mm/tlb: Flush remote and local TLBs
 concurrently

On 27/05/19 11:47, Peter Zijlstra wrote:
> On Sat, May 25, 2019 at 10:54:50AM +0200, Juergen Gross wrote:
>> On 25/05/2019 10:22, Nadav Amit wrote:
> 
>>> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
>>> index 946f8f1f1efc..3a156e63c57d 100644
>>> --- a/arch/x86/include/asm/paravirt_types.h
>>> +++ b/arch/x86/include/asm/paravirt_types.h
>>> @@ -211,6 +211,12 @@ struct pv_mmu_ops {
>>>  	void (*flush_tlb_user)(void);
>>>  	void (*flush_tlb_kernel)(void);
>>>  	void (*flush_tlb_one_user)(unsigned long addr);
>>> +	/*
>>> +	 * flush_tlb_multi() is the preferred interface. When it is used,
>>> +	 * flush_tlb_others() should return false.
>>
>> This comment does not make sense. flush_tlb_others() return type is
>> void.
> 
> I suspect that is an artifact from before the static_key; an attempt to
> make the pv interface less awkward.
> 
> Something like the below would work for KVM I suspect, the others
> (Hyper-V and Xen are more 'interesting').
> 
> ---
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -580,7 +580,7 @@ static void __init kvm_apf_trap_init(voi
>  
>  static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);
>  
> -static void kvm_flush_tlb_others(const struct cpumask *cpumask,
> +static void kvm_flush_tlb_multi(const struct cpumask *cpumask,
>  			const struct flush_tlb_info *info)
>  {
>  	u8 state;
> @@ -594,6 +594,9 @@ static void kvm_flush_tlb_others(const s
>  	 * queue flush_on_enter for pre-empted vCPUs
>  	 */
>  	for_each_cpu(cpu, flushmask) {
> +		if (cpu == smp_processor_id())
> +			continue;
> +

Even this would be just an optimization; the vCPU you're running on
cannot be preempted.  You can just change others to multi.

Paolo

>  		src = &per_cpu(steal_time, cpu);
>  		state = READ_ONCE(src->preempted);
>  		if ((state & KVM_VCPU_PREEMPTED)) {
> @@ -603,7 +606,7 @@ static void kvm_flush_tlb_others(const s
>  		}
>  	}
>  
> -	native_flush_tlb_others(flushmask, info);
> +	native_flush_tlb_multi(flushmask, info);
>  }
>  
>  static void __init kvm_guest_init(void)
> @@ -628,9 +631,8 @@ static void __init kvm_guest_init(void)
>  	if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
>  	    !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
>  	    kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
> -		pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
> +		pv_ops.mmu.flush_tlb_multi = kvm_flush_tlb_multi;
>  		pv_ops.mmu.tlb_remove_table = tlb_remove_table;
> -		static_key_disable(&flush_tlb_multi_enabled.key);
>  	}
>  
>  	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>