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Date:   Mon, 24 Jun 2019 12:43:30 +0100
From:   Dave Martin <Dave.Martin@arm.com>
To:     Marc Zyngier <marc.zyngier@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        kvm@vger.kernel.org, Julien Thierry <julien.thierry@arm.com>,
        Andre Przywara <andre.przywara@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Christoffer Dall <christoffer.dall@arm.com>,
        James Morse <james.morse@arm.com>,
        Jintack Lim <jintack@cs.columbia.edu>
Subject: Re: [PATCH 04/59] KVM: arm64: nv: Introduce nested virtualization
 VCPU feature
Message-ID: <20190624114329.GO2790@e103592.cambridge.arm.com>
References: <20190621093843.220980-1-marc.zyngier@arm.com>
 <20190621093843.220980-5-marc.zyngier@arm.com>
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On Fri, Jun 21, 2019 at 10:37:48AM +0100, Marc Zyngier wrote:
> From: Christoffer Dall <christoffer.dall@arm.com>
> 
> Introduce the feature bit and a primitive that checks if the feature is
> set behind a static key check based on the cpus_have_const_cap check.
> 
> Checking nested_virt_in_use() on systems without nested virt enabled
> should have neglgible overhead.
> 
> We don't yet allow userspace to actually set this feature.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---

[...]

> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> new file mode 100644
> index 000000000000..8a3d121a0b42
> --- /dev/null
> +++ b/arch/arm64/include/asm/kvm_nested.h
> @@ -0,0 +1,13 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ARM64_KVM_NESTED_H
> +#define __ARM64_KVM_NESTED_H
> +
> +#include <linux/kvm_host.h>
> +
> +static inline bool nested_virt_in_use(const struct kvm_vcpu *vcpu)
> +{
> +	return cpus_have_const_cap(ARM64_HAS_NESTED_VIRT) &&
> +		test_bit(KVM_ARM_VCPU_NESTED_VIRT, vcpu->arch.features);
> +}

Also, is it worth having a vcpu->arch.flags flag for this, similarly to
SVE and ptrauth?

[...]

Cheers
---Dave