Re: [PATCH RESEND v2 5/6] target/arm/cpu: Enable 'el2' to work with host/max cpu

[Peter Maydell]

On Tue, 27 Apr 2021 at 10:24, Andrew Jones <drjones@redhat.com> wrote:
> I feel like there are way too many ways to track this feature now. If I
> didn't lose count we have
>
>  1) cpu->has_el2
>  2) cpu->env & ARM_FEATURE_EL2
>  3) (for mach-virt) vms->virt
>  4) possibly (and probably should) some ID register bits
>
> I realize the first three are already in use for TCG, but I'm guessing
> we'll want to clean those up. What's the plan going forward? I presume
> it'll be (4), but maybe something like (1) and/or (3) will stick around
> for convenience. I'm pretty sure we want to avoid (2).

For new features added we want to use ID register bits. However,
a lot of older pre-existing features are keyed off ARM_FEATURE_FOO
flag bits. Trying to convert an ARM_FEATURE flag to use ID registers
isn't necessarily 100% trivial (for instance, the ARM_FEATURE flag
is always checkable regardless of AArch64 vs AArch32, but the ID
register checks often need to be split up into separate ones checking
the AArch32 or the AArch64 ID register value). So we aren't really
doing conversion of existing flags. (I did a few which were easy
because there were only a handful of checks of them.) As a side note,
some features really can't be checked in ID registers, like
ARM_FEATURE_V8_1M, so env->features is not going to go away completely.

The only use of cpu->has_foo should be for the QOM property -- the
arm_cpu_realizefn() should look at it and then either clear the
ARM_FEATURE flag or update the ID register bits depending on
which one the feature is using.

vms->virt is for the board code (and controls more things than
just whether the CPU itself has EL2).

thanks
-- PMM