Re: [RFC PATCH v6 03/35] KVM: arm64: Add CONFIG_KVM_ARM_SPE Kconfig option

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi James,

On Fri, Jan 09, 2026 at 04:29:50PM +0000, James Clark wrote:
> 
> 
> On 14/11/2025 4:06 pm, Alexandru Elisei wrote:
> > Add a new configuration option that will be used for KVM SPE emulation.
> > CONFIG_KVM_ARM_SPE depends on the SPE driver being builtin because:
> > 
> > 1. The SPE driver maintains a cpumask of physical CPUs that support SPE,
> >     and that will be used by KVM to emulate SPE on heterogeneous systems.
> > 
> > 2. KVM will rely on the SPE driver enabling the SPE interrupt at the GIC
> >     level.
> > 
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> >   arch/arm64/kvm/Kconfig | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> > index 4f803fd1c99a..31388b5b2655 100644
> > --- a/arch/arm64/kvm/Kconfig
> > +++ b/arch/arm64/kvm/Kconfig
> > @@ -83,4 +83,12 @@ config PTDUMP_STAGE2_DEBUGFS
> >   	  If in doubt, say N.
> > +config KVM_ARM_SPE
> > +	bool
> > +	depends on KVM && ARM_SPE_PMU=y
> 
> I think the most common configuration is module, so requiring built-in isn't
> great. If there's any way of avoiding it, even if it costs a little bit of
> pain, it would be good for adoption.

I'm not sure how that could be done. You need the buffer maintenance interrupt
to do a VM exit so KVM can inject the virtual interrupt back into the guest,
otherwise there's a possibility of a large blackout window when the buffer is
full.

The code also relies on the SPE driver to probe all the SPUs in the system.

> 
> It was basically the first thing I got stuck on trying to get it to work. Or
> if it really can't be done, maybe some kind of debug message to say why
> enabling failed.

When you search for the symbol in menuconfig you get a list of the symbols it
depends on.

Thanks,
Alex