Re: [PATCH v2 0/7] arm64: KVM: vgic-v2: Allow unsafe GICV accesses

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

On Tue, Sep 06, 2016 at 09:28:40AM +0100, Marc Zyngier wrote:
> In a number of cases, KVM cannot give access direct access to the
> GICv2 GICV region, either because GICV is not page aligned, or its
> size is not a multiple of the page size. This is especially visible
> with 16kB/64kB pages and the original GIC-400 layout where each region
> is only 4k aligned.
> 
> Instead of disabling KVM altogether (which is the current behaviour),
> there is some value in trapping each guest GICV access, performing the
> access as quickly as possible at EL2, and resuming the guest. This
> allows us to keep KVM enabled on this HW.
> 
> Implementation wise, this is done with a static key controlling the
> workaround being enabled, hence coming at zero cost (well, an extra
> nop on the exit hot path) for unaffected platforms. On the affected
> HW, I've measured a 10 to 15% overhead for a self-IPI test, which is
> pretty bad, but still much better than not having a GIC at all.
> 
> There is two pending issues:
> 
> - A failed write to GICV ends up being forwarded to userspace. This
>   will be addressed in a follow-up series where we deal with injecting
>   vSError in the guest
> 
> - Skipping instructions (as we do when emulating anything) breaks
>   things like guest single-step and watchpoints. This is a long
>   standing problem, and someone should probably have a look at
>   it. Alex?
> 
> Tested on Juno-r1 with 64kB pages.

I also tested this on TC2 to ensure we didn't regress the 32-bit side.

Applied the series.
-Christoffer