Re: [PATCH kvmtool v4 5/7] arm64: Add FEAT_E2H0 support

[Marc Zyngier <maz@kernel.org>]

On Fri, 16 Jan 2026 18:12:40 +0000,
Sascha Bischoff <Sascha.Bischoff@arm.com> wrote:
> 
> On Wed, 2025-09-24 at 14:45 +0100, Andre Przywara wrote:
> > From: Marc Zyngier <maz@kernel.org>
> > 
> > The --nested option allows a guest to boot at EL2 without FEAT_E2H0
> > (i.e. mandating VHE support). While this is great for "modern"
> > operating
> > systems and hypervisors, a few legacy guests are stuck in a distant
> > past.
> > 
> > To support those, add the --e2h0 command line option, that exposes
> > FEAT_E2H0 to the guest, at the expense of a number of other features,
> > such
> > as FEAT_NV2. This is conditioned on the host itself supporting
> > FEAT_E2H0.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> 
> One inline comment below, but irrespective:
> 
> Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
[...]

> > diff --git a/arm64/kvm-cpu.c b/arm64/kvm-cpu.c
> > index 42dc11dad..5e4f3a7dd 100644
> > --- a/arm64/kvm-cpu.c
> > +++ b/arm64/kvm-cpu.c
> > @@ -76,6 +76,11 @@ static void kvm_cpu__select_features(struct kvm
> > *kvm, struct kvm_vcpu_init *init
> >  		if (!kvm__supports_extension(kvm, KVM_CAP_ARM_EL2))
> >  			die("EL2 (nested virt) is not supported");
> >  		init->features[0] |= 1UL << KVM_ARM_VCPU_HAS_EL2;
> > +		if (kvm->cfg.arch.e2h0) {
> > +			if (!kvm__supports_extension(kvm,
> > KVM_CAP_ARM_EL2_E2H0))
> > +				die("FEAT_E2H0 is not supported");
> > +			init->features[0] |= 1UL <<
> > KVM_ARM_VCPU_HAS_EL2_E2H0;
> > +		}
> 
> --e2h0 is only consumed if --nested is also set (correctly so, by my
> understanding & the KVM docs).
> 
> Maybe it is worth printing that it isn't consumed without --nested if
> only --e2h0 is supplied? Avoids the user thinking that it has an effect
> when it doesn't. (Alternatively, make --e2h0 imply --nested, but I'm
> not a fan of that, personally.)

Sure, I can add a warning indicating that the option is ignored.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.