Re: [PATCH 01/16] KVM: arm64: Generalise VM features into a set of flags

From: Will Deacon <will@kernel.org>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, qperret@google.com,
	dbrazdil@google.com, Srivatsa Vaddagiri <vatsa@codeaurora.org>,
	Shanker R Donthineni <sdonthineni@nvidia.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH 01/16] KVM: arm64: Generalise VM features into a set of flags
Date: Tue, 27 Jul 2021 19:10:27 +0100	[thread overview]
Message-ID: <20210727181026.GA19173@willie-the-truck> (raw)
In-Reply-To: <20210715163159.1480168-2-maz@kernel.org>

On Thu, Jul 15, 2021 at 05:31:44PM +0100, Marc Zyngier wrote:
> We currently deal with a set of booleans for VM features,
> while they could be better represented as set of flags
> contained in an unsigned long, similarily to what we are
> doing on the CPU side.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_host.h | 12 +++++++-----
>  arch/arm64/kvm/arm.c              |  5 +++--
>  arch/arm64/kvm/mmio.c             |  3 ++-
>  3 files changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 41911585ae0c..4add6c27251f 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -122,7 +122,10 @@ struct kvm_arch {
>  	 * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is
>  	 * supported.
>  	 */
> -	bool return_nisv_io_abort_to_user;
> +#define KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER	0
> +	/* Memory Tagging Extension enabled for the guest */
> +#define KVM_ARCH_FLAG_MTE_ENABLED			1
> +	unsigned long flags;

One downside of packing all these together is that updating 'flags' now
requires an atomic rmw sequence (i.e. set_bit()). Then again, that's
probably for the best anyway given that kvm_vm_ioctl_enable_cap() looks
like it doesn't hold any locks.

>  	/*
>  	 * VM-wide PMU filter, implemented as a bitmap and big enough for
> @@ -133,9 +136,6 @@ struct kvm_arch {
>  
>  	u8 pfr0_csv2;
>  	u8 pfr0_csv3;
> -
> -	/* Memory Tagging Extension enabled for the guest */
> -	bool mte_enabled;
>  };
>  
>  struct kvm_vcpu_fault_info {
> @@ -777,7 +777,9 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu *vcpu);
>  #define kvm_arm_vcpu_sve_finalized(vcpu) \
>  	((vcpu)->arch.flags & KVM_ARM64_VCPU_SVE_FINALIZED)
>  
> -#define kvm_has_mte(kvm) (system_supports_mte() && (kvm)->arch.mte_enabled)
> +#define kvm_has_mte(kvm)					\
> +	(system_supports_mte() &&				\
> +	 test_bit(KVM_ARCH_FLAG_MTE_ENABLED, &(kvm)->arch.flags))

Not an issue with this patch, but I just noticed that the
system_supports_mte() check is redundant here as we only allow the flag to
be set if that's already the case.

Will