[RFC PATCH 6/7] arm64: KVM: Handle trappable TLB instructions

Will Deacon [off-list ref] writes:

On Fri, Aug 26, 2016 at 10:37:08AM +0100, Punit Agrawal wrote:

quoted

quoted

Will Deacon [off-list ref] writes:

quoted

The easiest thing to do is just TLBI VMALLE1IS for all trapped operations,
but you might want to see how that performs.

That sounds reasonable for correctness. But I suspect we'll have to do
more to claw back some performance. Let me run a few tests and come back
on this.

Assuming I've correctly switched in TCR and replacing the various TLB
operations in this patch with TLBI VMALLE1IS, there is a drop in kernel
build times of ~5% (384s vs 363s).

What do you mean by "switched in TCR"? Why is that necessary if you just
nuke the whole thing?

You're right. it's not necessary. I'd misunderstood how TCR affects
things and was switching it in the above tests.

Is the ~5% relative to no trapping at all, or
trapping, but being selective about the operation?

The reported number was relative to trapping and being selective about
the operation. But I hadn't been careful in ensuring identical
conditions (page caches, etc.) when running the numbers.

So I've done a fresh set of identical measurements by running "time make
-j 7" in a VM booted with 7 vcpus and see the following results

1. no trapping ~ 365s
2. traps using selective tlb operations ~ 371s
3. traps that nuke all stage 1 (tlbi vmalle1is) ~ 393s

So based on these measurements there is ~1% and ~7.5% drop in comparison
between 2. and 3. compared to the base case of no trapping at all.

Thanks,
Punit

Will
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