Re: [PATCH v15 12/37] KVM: arm64: CCA: Support the VGIC in realms

[Steven Price]

On 23/07/2026 07:56, Kohei Enju wrote:
> On 07/22 14:31, Steven Price wrote:
>> On 22/07/2026 09:27, Kohei Enju wrote:
>>> On 07/15 15:28, Steven Price wrote:
>>>> The RMM provides emulation of a VGIC to the realm guest. With RMM v2.0
>>>> the registers are passed in the system registers so this works similar
>>>> to a normal guest, but kvm_arch_vcpu_put() need reordering to early out,
>>>> and realm guests don't support GICv2 even if the host does.
>>>>
>>>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>>>
>>> Hi Steven,
>>
>> Hi Kohei,
>>
>>> I've been testing this series and found that when the host CPU doesn't have
>>> ARM64_HAS_ICH_HCR_EL2_TDIR this series doesn't work as expected.
>>
>> Thanks for testing!
>>
>>> Since commit 2a28810cbb8b ("KVM: arm64: GICv3: Detect and work around the lack
>>> of ICV_DIR_EL1 trapping"), when the host CPU doesn't support this feature, KVM
>>> traps all GIC sysreg accesses in the common group. However, currently trap
>>> handlers for ICC_{PMR,RPR,CTLR}_EL1 registers are missing [0]. So when Realm
>>> guests try to access those registers, KVM traps them but just emits the warning
>>> shown in [1], and Realm guests fail to boot.
>>>
>>> As far as I can tell, the CCA requirements don't require the
>>> ARM64_HAS_ICH_HCR_EL2_TDIR feature. If that's the case, this seems to be a
>>> problem. Is there any workaround for this issue, or should we implement trap
>>> handlers for those registers?
>>
>> As Marc has already said in his reply, I'm really surprised you have a
>> CPU which implements CCA but doesn't have ARM64_HAS_ICH_HCR_EL2_TDIR.
>> Can you give some more details about the platform you are testing on?
> 
> Thank you for taking a look, Steve.
> 
> Unfortunately, due to the company's policy, I can't share any details
> about the platform I'm currently testing on. When the time is right,
> I'll be happy to do so.
> 
>>
>> Ultimately there are two options here - either don't support CCA (so
>> detect the lack of ARM64_HAS_ICH_HCR_EL2_TDIR and bail out early), or
>> plumb in the trap handlers - as commit 2a28810cbb8b points out this
>> isn't something we really want to support if we can avoid it.
> 
> Yes, the former makes perfect sense if there would be no systems that
> supports CCA without TDIR. However, if such systems do exist, I'd be
> interested in exploring the latter approach.
> 
>>
>> Hence I'm interested to know where this sits between "hacked up test
>> system" and "production hardware". E.g. if this is an emulator it might
>> be possible to just enable the CPU feature.
> 
> Again, I can't share that right now, but I'd like to share more when the
> time comes.

Given Marc's response for now I'm not going to add support for this. 
"When the time comes" we can revisit whether it makes sense to have 
upstream support and what that might look like.

However, I did ask an AI tool to have a go at implementing this, and it 
came up with the (very lightly tested) patch below. That might at least 
give you something you can test. It doesn't meet Marc's request that it
reuses the existing implementation, and it took the AI a few goes at
getting something that "works" so I suspect it might have bugs - I
haven't reviewed the code myself.

Thanks,
Steve

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