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

[Christoffer Dall]

On Tue, Feb 16, 2016 at 08:05:29PM +0000, Marc Zyngier wrote:
> On 10/02/16 20:40, Christoffer Dall wrote:
> > 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.
> 
> I've ran a slightly heavier set of tests, and the infamous reboot loop
> broke, thanks to patch #7.
> 
> Notice how we fail to wipe the vgic_apr copy on the "light" exit path?
> If you're unlucky (and odds are that you will be), you will inject an
> interrupt while its active priority bit is set, and the new interrupt
> won't be delivered. Bah.
> 
> With that fixed, the reboot loop has been going strong for a few hours.
> I'll leave my Seattle cooking overnight and if everything looks good in
> the morning, I'll repost a new set of patches.
> 
Good that you caught this one then!

-Christoffer