Re: [PATCH v2 0/3] arm: demuxed ID registers (CCSIDR_EL1)

[Cornelia Huck <cohuck@redhat.com>]

On Thu, Feb 05 2026, Peter Maydell <peter.maydell@linaro.org> wrote:

> On Wed, 4 Feb 2026 at 13:32, Cornelia Huck <cohuck@redhat.com> wrote:
>> RFC because there are still some open questions:
>
>> - There's a slight disagreement between the current code (providing 16
>>   entries for CCSIDR_EL1) and the KVM code (providing (7 cache levels) *
>>   (data/unified, instruction) = 14 entries.) With FEAT_MTE2, we might be
>>   needing 7 more entries.
>
> This is because architecturally the CCSELR_EL1 register is:
>  bit 4 : TnD (tag-not-data)
>  bits [3:1] : level
>  bit 0 : InD (instruction-not-data)
>
> but the 'level' field has 0b111 as Reserved, and KVM doesn't
> (currently) handle the FEAT_MTE2 TnD bit, so from KVM's point
> of view only 0..13 are valid values here.
>
> The architecture says that if you set CSSELR_EL1 to an invalid
> setting then reading CCSIDR_EL1 can be:
>  * a NOP
>  * UNDEF
>  * return an UNKNOWN value
>
> For TCG we opted to permit any indexes 0..15, so that we treat
> the architecturally invalid 0b1110 and 0b1111 the same way as
> we treat "you picked a level this particular CPU doesn't implement"
> (and return a 0 from our cpu->cssidr[] array). Otherwise we would
> have had to add an extra check for "and level isn't invalid".

That makes sense.

>
> TCG also doesn't do anything with the FEAT_MTE2 TnD bit: looks
> like we missed that functionality when we added MTE2 emulation.
> Setting TnD and InD at once isn't a valid combination, so we
> could either extend our cpu->ccsidr[] array to 32 words (and
> live with 8 of the extras being unused), or else add a new
> 8-word array for the MTE tag-cache ID values.

Whatever we do, it would be easiest if KVM and TCG followed a similar
approach, so we don't end up having to special case the code.

As KVM also supports setting (virtual) CCSIDR_EL1 values, I think just
extending the array would be easiest, so that QEMU can just keep
everything in one place.