linux-kernel - Re: [PATCH v3 4/4] KVM: Add flush_on

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Date:   Fri, 10 Nov 2017 11:15:06 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Radim Kr??m???? <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v3 4/4] KVM: Add flush_on_enter before guest enter

On 10/11/2017 11:08, Peter Zijlstra wrote:
> On Fri, Nov 10, 2017 at 01:49:47AM -0800, Wanpeng Li wrote:
>> @@ -2887,7 +2899,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
>>  	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
>>  		return;
>>  
>> -	vcpu->arch.st.steal.preempted = KVM_VCPU_PREEMPTED;
>> +	vcpu->arch.st.steal.preempted |= KVM_VCPU_PREEMPTED;
> 
> I don't understand this one... If there is concurrency its wrong, if
> there is no concurrency it still doesn't make sense as there should not
> be any FLUSH flags to preserve..

There is no concurrency, foreign VCPUs are not going to write to the
location unless PREEMPTED is set.  So indeed the "|=" is pointless.

However, I wonder if it'd be useful for a VCPU to set the bit _on
itself_ before going to sleep.  Like

    set KVM_VCPU_SHOULD_FLUSH
    hlt
    /* Automagic TLB flush!  */

This would not work currently, but if it *is* useful, we should make it
work and document it as legal.  Peter, do you think it would make any sense?

Thanks,

Paolo


>>  
>>  	kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.st.stime,
>>  			&vcpu->arch.st.steal.preempted,