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

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Message-ID: <81a67fa3-309d-79cc-5009-5c4908b18ba3@redhat.com>
Date:   Mon, 27 May 2019 14:45:17 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Juergen Gross <jgross@...e.com>, 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 14:32, Peter Zijlstra wrote:
> On Mon, May 27, 2019 at 12:21:59PM +0200, Paolo Bonzini wrote:
>> On 27/05/19 11:47, Peter Zijlstra wrote:
> 
>>> --- 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.
> 
> Yeah, I know, but it felt weird so I added the explicit skip. No strong
> feelings though.

Neither here, and it would indeed deserve a comment if you left the if out.

Paolo