[PATCH v6 11/25] KVM: arm64: Add remaining ID registers to id_reg_desc_table

[Oliver Upton]

Hi Reiji,

On Thu, Mar 10, 2022 at 08:47:57PM -0800, Reiji Watanabe wrote:
> Add hidden or reserved ID registers, and remaining ID registers,
> which don't require special handling, to id_reg_desc_table.
> Add 'flags' field to id_reg_desc, which is used to indicates hiddden
> or reserved registers. Since now id_reg_desc_init() is called even
> for hidden/reserved registers, change it to not do anything for them.
> 
> Signed-off-by: Reiji Watanabe <reijiw at google.com>

I think there is a very important detail of the series that probably
should be highlighted. We are only allowing AArch64 feature registers to
be configurable, right? AArch32 feature registers remain visible with
their default values passed through to the guest. If you've already
stated this as a precondition elsewhere then my apologies for the noise.

I don't know if adding support for this to AArch32 registers is
necessarily the right step forward, either. 32 bit support is working
just fine and IMO its OK to limit new KVM features to AArch64-only so
long as it doesn't break 32 bit support. Marc of course is the authority
on that, though :-)

If for any reason a guest uses a feature present in the AArch32 feature
register but hidden from the AArch64 register, we could be in a
particularly difficult position. Especially if we enabled traps based on
the AArch64 value and UNDEF the guest.

One hack we could do is skip trap configuration if AArch32 is visible at
either EL1 or EL0, but that may not be the most elegant solution.
Otherwise, if we are AArch64-only at every EL then the definition of the
AArch32 feature registers is architecturally UNKNOWN, so we can dodge
the problem altogether. What are your thoughts?

--
Thanks,
Oliver