linux-kernel - Re: [RFC PATCH 06/28] arm64: RME: ioctls to create and configure realms

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Date: Mon, 18 Mar 2024 11:22:39 +0000
From: Steven Price <steven.price@....com>
To: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 kvm@...r.kernel.org, kvmarm@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 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
Subject: Re: [RFC PATCH 06/28] arm64: RME: ioctls to create and configure
 realms

Thanks for taking a look at this.

On 18/03/2024 07:40, Ganapatrao Kulkarni wrote:
> On 27-01-2023 04:59 pm, Steven Price wrote:
[...]
>>   int kvm_init_rme(void)
>>   {
>> +    int ret;
>> +
>>       if (PAGE_SIZE != SZ_4K)
>>           /* Only 4k page size on the host is supported */
>>           return 0;
>> @@ -43,6 +394,12 @@ int kvm_init_rme(void)
>>           /* Continue without realm support */
>>           return 0;
>>   +    ret = rme_vmid_init();
>> +    if (ret)
>> +        return ret;
>> +
>> +    WARN_ON(rmi_features(0, &rmm_feat_reg0));
> 
> Why WARN_ON, Is that good enough to print err/info message and keep
> "kvm_rme_is_available" disabled?

Good point. RMI_FEATURES "does not have any failure conditions" so this
is very much a "should never happen" situation. Assuming the call
gracefully fails then rmm_feat_reg0 would remain 0 which would in
practise stop realms being created, but this is clearly non-ideal.

I'll fix this up in the next version to do the rmi_features() call
before rme_vmid_init(), that way we can just return early without
setting kvm_rme_is_available in this situation. I'll keep the WARN_ON
because something has gone very wrong if this call fails.

> IMO, we should print message when rme is enabled, otherwise it should be
> silent return.

The rmi_check_version() call already outputs a "RMI ABI version %d.%d"
message - I don't want to be too noisy here. Other than the 'cannot
happen' situations if you see the "RMI ABI" message then
kvm_rme_is_available will be set. And those 'cannot happen' routes will
print their own error message (and point to a seriously broken system).

And obviously in the case of SMC_RMI_VERSION not being supported then we
silently return as this is taken to mean there isn't an RMM.

Thanks,

Steve