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

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Message-ID: <20171110103104.h2ygyynvljvquiv6@hirez.programming.kicks-ass.net>
Date:   Fri, 10 Nov 2017 11:31:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, 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 Fri, Nov 10, 2017 at 11:15:06AM +0100, Paolo Bonzini wrote:
> 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?

Almost but not quite I think.. So there is no guarantee HLT (or even
MWAIT with a state that has CPUILDE_FLAG_TLB_FLUSHED set) will actually
do the TLB flush.

And if we preempt the vCPU to run a kernel thread we will not in fact
invalidate the TLBs either.

Also, you're confusing the SHOULD_FLUSH with the HAS_FLUSHED concept.
Because if we didn't flush and we should have we should still issue it
on VMENTER.

So if we could somehow tell if a HLT or preemption did indeed flush the
TLBs post fact (reading back the attained C state is possible but really
rather expensive IIRC), then we could set a HAS_FLUSHED flag and avoid
issuing when SHOULD_FLUSH is also set.