Re: [RFC PATCH v2 18/28] KVM: arm64: Introduce KVM_CAP_ARM_ID_REG_WRITABLE capability

From: Oliver Upton <oupton@google.com>
To: Reiji Watanabe <reijiw@google.com>
Cc: Marc Zyngier <maz@kernel.org>,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Will Deacon <will@kernel.org>, Andrew Jones <drjones@redhat.com>,
	Peng Liang <liangpeng10@huawei.com>,
	Peter Shier <pshier@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>
Subject: Re: [RFC PATCH v2 18/28] KVM: arm64: Introduce KVM_CAP_ARM_ID_REG_WRITABLE capability
Date: Thu, 4 Nov 2021 16:40:58 +0000	[thread overview]
Message-ID: <YYQNGqpy1NiUEXYD@google.com> (raw)
In-Reply-To: <20211103062520.1445832-19-reijiw@google.com>

On Tue, Nov 02, 2021 at 11:25:10PM -0700, Reiji Watanabe wrote:
> Introduce a new capability KVM_CAP_ARM_ID_REG_WRITABLE to indicate
> that ID registers are writable by userspace.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
>  Documentation/virt/kvm/api.rst | 8 ++++++++
>  arch/arm64/kvm/arm.c           | 1 +
>  include/uapi/linux/kvm.h       | 1 +
>  3 files changed, 10 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index a6729c8cf063..f7dfb5127310 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7265,3 +7265,11 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
>  of the result of KVM_CHECK_EXTENSION.  KVM will forward to userspace
>  the hypercalls whose corresponding bit is in the argument, and return
>  ENOSYS for the others.
> +
> +8.35 KVM_CAP_ARM_ID_REG_WRITABLE
> +--------------------------------

ID registers are technically already writable, KVM just rejects any
value other than what it derives from sanitising the host ID registers.
I agree that the nuance being added warrants a KVM_CAP, as it informs
userspace it can deliberately configure ID registers with a more limited
value than what KVM returns.

KVM_CAP_ARM_ID_REG_CONFIGURABLE maybe? Naming is hard :)

--
Thanks,
Oliver