[MODERATED] Re: L1D-Fault KVM mitigation

[Andi Kleen <ak@linux.intel.com>]

On Sun, May 27, 2018 at 05:42:46PM +0200, speck for Thomas Gleixner wrote:
> Whatever the guest mode does is irrelevant.

Idle in the guest means already exited to the host because
HLT exit is always enabled.

> 
> and this ping pong goes on 10 times in a row taking 2+ milliseconds and the
> progress made is the guest is minimal. The reason for this are interrupts
> targeted to one of the vCPUs or operations in one of the guest threads
> which cause several exits in a row. And there is nothing you can do about
> that. It's completely workload dependent.

There's a threshold of the exit rate somewhere where HT off is probably 
better, but I don't think we really have characterized well so far where
exactly this threshold is.

My suspicion is that a lot of workloads will be below the threshold.
We'll see what works out.

> 
> So unless you have a fully controlled scenario where the guests almost
> never exit, the whole synchronization stuff is doomed. But fully controlled
> means a 1:1 relationship of physical and virtual CPUs like David mentioned.
> Yes, that stuff can benefit, but then we rather want ucode assistance than
> the whole wait/IPI dance in software.

Right that would be an obvious optimization. However in the traces
I looked so far the IPI was actually not the most expensive part.

I think it's because IPIs between siblings are not very expensive.

-Andi