Re: [PATCH 0/8] KVM/ARM: Guest Entry/Exit optimizations

[Christoffer Dall <christoffer.dall@linaro.org>]

On Mon, Feb 08, 2016 at 11:40:14AM +0000, Marc Zyngier wrote:
> I've recently been looking at our entry/exit costs, and profiling
> figures did show some very low hanging fruits.
> 
> The most obvious cost is that accessing the GIC HW is slow. As in
> "deadly slow", specially when GICv2 is involved. So not hammering the
> HW when there is nothing to write is immediately beneficial, as this
> is the most common cases (whatever people seem to think, interrupts
> are a *rare* event).
> 
> Another easy thing to fix is the way we handle trapped system
> registers. We do insist on (mostly) sorting them, but we do perform a
> linear search on trap. We can switch to a binary search for free, and
> get immediate benefits (the PMU code, being extremely trap-happy,
> benefits immediately from this).
> 
> With these in place, I see an improvement of 20 to 30% (depending on
> the platform) on our world-switch cycle count when running a set of
> hand-crafted guests that are designed to only perform traps.
> 

By the way, I took this whole stack of changes (wsinc, vhe, and
optimizations) and ran it on Mustang and fired up UEFI and did a reboot
and things seem to work, so that's a small shallow
'tested-by-something-else-than-a-linux-guest' statement from me.

-Christoffer