[PATCH v2 3/3] KVM: arm64: Start trapping ID registers for 32 bit guests

[Reiji Watanabe]

On Thu, Mar 31, 2022 at 6:08 PM Oliver Upton <oupton at google.com> wrote:
>
> To date KVM has not trapped ID register accesses from AArch32, meaning
> that guests get an unconstrained view of what hardware supports. This
> can be a serious problem because we try to base the guest's feature
> registers on values that are safe system-wide. Furthermore, KVM does not
> implement the latest ISA in the PMU and Debug architecture, so we
> constrain these fields to supported values.
>
> Since KVM now correctly handles CP15 and CP10 register traps, we no
> longer need to clear HCR_EL2.TID3 for 32 bit guests and will instead
> emulate reads with their safe values.
>
> Signed-off-by: Oliver Upton <oupton at google.com>

Reviewed-by: Reiji Watanabe <reijiw at google.com>

BTW, due to this, on a system that supports PMUv3, ID_DFR0_E1 value will
become 0 for the aarch32 guest without PMUv3. This is the correct behavior,
but it affects migration.  I'm not sure how much we should care about
migration of the aarch32 guest though (and it will be resolved once ID
registers become configurable anyway).

Thanks,
Reiji