Re: [RFC PATCH 0/7] KVM: arm/arm64: Optimize arch timer register handling

Christoffer Dall
2017-01-06T10:00:14Z

On Sat, Dec 10, 2016 at 09:47:05PM +0100, Christoffer Dall wrote:
> We currently spend around ~400 cycles on each entry/exit to the guest
> dealing with arch timer registers, even when the timer is not pending
> and not doing anything.
>
> We can do much better by moving the arch timer save/restore to the
> vcpu_load and vcpu_put functions, but this means that if we don't read
> back the timer state on every exit from the guest, then we have to be
> able to start taking timer interrupts for the virtual timer in KVM and
> handle that properly.
>
> That has a number of funny consequences, such as having to make sure we
> don't deadlock between any of the vgic code and interrupt injection
> happening from an ISR. On the plus side, being able to inject
> virtual interrupts corresponding to a physical interrupt directly from
> an ISR is probably a good system design change.
>
> We also have to change the use of the physical vs. virtual counter in
> the arm64 kernel to avoid having to save/restore the CNTVOFF_EL2
> register on every return to the hypervisor. The only reason I could
> find for using the virtual counter for the kernel on systems with access
> to the physical counter is to detect if firmware did not properly clear
> CNTVOFF_EL2, and this change has to weighed against the existing check
> (assuming I got this right).
>
> On a non-VHE system (AMD Seattle) I have measured this to improve the
> world-switch time by about ~100 cycles, but on an EL2 kernel (emulating
> VHE behavior on the same hardware) this gives us around ~250 cycles
> worth of improvement, because we can avoid the extra configuration of
> trapping accesses to the physical timer from EL1 on every switch.
>
> I'm not sure if the benefits outweigh the complexity of this patch set,
> nor am I sure if I'm missing an overall better approach, hence the RFC
> tag on the series.
>
> I'm looking forward to overall comments on the approach.
>

FYI for anyone looking at these patches, I found some issues with the
series and will respin shortly. Patch 5 has also been split to
hopefully simplify the review, as I realized it is horrible to look at
in its current form.

Thanks,
-Christoffer