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Date: Sun, 27 May 2018 11:25:50 -0700
From: Andi Kleen <ak@linux.intel.com>
Subject: [MODERATED] Re: L1D-Fault KVM mitigation
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To: speck@linutronix.de
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On Sat, May 26, 2018 at 01:48:01PM -0700, speck for Linus Torvalds wrote:
> 
> 
> On Sat, 26 May 2018, speck for Andi Kleen wrote:
> > 
> > Keep in mind that we don't need to synchronize when the other CPU is idle
> > in the guest, so it's only a problem when all the CPUs are busy.
> 
> What? No.
> 
> Maybe you mean "idle in the _host_"?

It's both actually. Either idle in the host, or idle in the guest.

When it's idle in the host we don't need to synchronize, unless 
there is an interrupt (which does its own synchronization) because
the idle loop has nothing valuable to leak.

And if it's idle in the guest then the vcpu is blocked and
also obviously doesn't need to be synchronized.

Both cases can be reasonably common.

-Andi