linux-kernel - Re: [PATCH v12 6/8] arm64: kvm: Expose KVM_ARM_CAP

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Date:   Wed, 19 May 2021 14:26:31 +0100
From:   Steven Price <steven.price@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v12 6/8] arm64: kvm: Expose KVM_ARM_CAP_MTE

On 17/05/2021 18:40, Marc Zyngier wrote:
> On Mon, 17 May 2021 13:32:37 +0100,
> Steven Price <steven.price@....com> wrote:
>>
>> It's now safe for the VMM to enable MTE in a guest, so expose the
>> capability to user space.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>>  arch/arm64/kvm/arm.c      | 9 +++++++++
>>  arch/arm64/kvm/sys_regs.c | 3 +++
>>  2 files changed, 12 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 1cb39c0803a4..e89a5e275e25 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -93,6 +93,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>>  		r = 0;
>>  		kvm->arch.return_nisv_io_abort_to_user = true;
>>  		break;
>> +	case KVM_CAP_ARM_MTE:
>> +		if (!system_supports_mte() || kvm->created_vcpus)
>> +			return -EINVAL;
>> +		r = 0;
>> +		kvm->arch.mte_enabled = true;
> 
> As far as I can tell from the architecture, this isn't valid for a
> 32bit guest.

Indeed, however the MTE flag is a property of the VM not of the vCPU.
And, unless I'm mistaken, it's technically possible to create a VM where
some CPUs are 32 bit and some 64 bit. Not that I can see much use of a
configuration like that.

Steve