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[Tim Chen <tim.c.chen@linux.intel.com>]

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From: Tim Chen <tim.c.chen@linux.intel.com>
To: speck for Thomas Gleixner <speck@linutronix.de>
Subject: Re: L1D-Fault KVM mitigation

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On 05/26/2018 12:14 PM, speck for Thomas Gleixner wrote:
> On Thu, 24 May 2018, speck for Tim Chen wrote:
> 
 of load time.
>>
>> We may need to do the co-scheduling only when VM exit rate is low, and
>> turn off the SMT when VM exit rate becomes too high.
> 
> You cannot do that during runtime. That will destroy placement schemes and
> whatever. The SMT off decision needs to be done at a quiescent moment,
> i.e. before starting VMs.

Taking the SMT offline is a bit much and too big a hammer.  Andi and I thought about
having the scheduler forcing the other thread in idle instead for high
VM exit rate scenario. We don't have
to bother about doing sync with the other idle thread.

But we have issues about fairness, as we will be starving the
other run queue.

> 

> Running the same compile single threaded (offlining vCPU1 in the guest)
> increases the time to 107 seconds.
> 
>     107 / 88  = 1.22
> 
> I.e. it's 20% slower than the one using two threads. That means that it is
> the same slowdown as having two threads synchronized (your number).

yes, with compile workload, the HT speedup was mostly eaten up by
overhead.

> 
> So if I take the above example and assume that the overhead of
> synchronization is ~20% then the average vmenter/vmexit time is close to
> 50us.
> 

> 
> So I can see the usefulness for scenarious which David Woodhouse described
> where vCPU and host CPU have a fixed relationship and the guests exit once
> in a while. But that should really be done with ucode assisantance which
> avoids all the nasty synchronization hackery more or less completely.

The ucode guys are looking into such possibilities.  It is tough as they
have to work within the constraint of limited ucode headroom.

Thanks.

Tim