linux-kernel - Re: [PATCH v10 05/43] arm64: RME: Check for RME support at KVM init

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Message-ID: <99de8b04-7e7d-404b-aab8-5759a1f28c1a@arm.com>
Date: Wed, 1 Oct 2025 16:34:59 +0100
From: Steven Price <steven.price@....com>
To: Marc Zyngier <maz@...nel.org>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 James Morse <james.morse@....com>, Oliver Upton <oliver.upton@...ux.dev>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Joey Gouly <joey.gouly@....com>,
 Alexandru Elisei <alexandru.elisei@....com>,
 Christoffer Dall <christoffer.dall@....com>, Fuad Tabba <tabba@...gle.com>,
 linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 Gavin Shan <gshan@...hat.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 Alper Gun <alpergun@...gle.com>, "Aneesh Kumar K . V"
 <aneesh.kumar@...nel.org>, Emi Kisanuki <fj0570is@...itsu.com>,
 Vishal Annapurve <vannapurve@...gle.com>
Subject: Re: [PATCH v10 05/43] arm64: RME: Check for RME support at KVM init

On 01/10/2025 14:35, Marc Zyngier wrote:
> On Wed, 01 Oct 2025 14:20:13 +0100,
> Steven Price <steven.price@....com> wrote:
>>
>>>> +static int rmi_check_version(void)
>>>> +{
>>>> +	struct arm_smccc_res res;
>>>> +	unsigned short version_major, version_minor;
>>>> +	unsigned long host_version = RMI_ABI_VERSION(RMI_ABI_MAJOR_VERSION,
>>>> +						     RMI_ABI_MINOR_VERSION);
>>>> +
>>>> +	arm_smccc_1_1_invoke(SMC_RMI_VERSION, host_version, &res);
>>>
>>> Shouldn't you first check that RME is actually available, by looking
>>> at ID_AA64PFR0_EL1.RME?
>>
>> Well, you made a good point above that this isn't RME, it's CCA. And I
>> guess there's a possible world where the CCA interface could be
>> supported with something other than FEAT_RME (FEAT_RME2 maybe?) so I'm
>> not sure it necessarily a good idea to pin this on a CPU feature
>> bit.
> 
> But you cannot have CCA without RME. You cannot have CCA with
> GICv3. And my point was more that RME could be used by something other
> than CCA  - I certainly don't anticipate someone else adopting the CCA
> interface for anything...
> 
>> Ultimately what we want to know is whether the firmware thinks it can
>> supply us with the CCA interface and we don't really care how it
>> achieves it.
> 
> I disagree. You rely on specific feature sets to be available (hell,
> everything is baked around GICv3... GICv5 anyone?).
> 
> For this sort of stuff, you absolutely need to know what you are
> running on, not what some broken firmware tries to pretend it is.

Well I don't agree, but equally I think the chances of the RMM interface
existing without RME is nil, so I guess the extra check doesn't really
matter. So I'll add it. In the (highly) unlikely event that it causes a
problem then it would be easy enough to remove.

Thanks,
Steve