Re: [PATCH v2 1/2] arm64: Relax constraints on ID feature bits

[Will Deacon <will.deacon@arm.com>]

On Wed, Feb 07, 2018 at 02:21:05PM +0000, Suzuki K Poulose wrote:
> We treat most of the feature bits in the ID registers as STRICT,
> implying that all CPUs should match it the boot CPU state. However,
> for most of the features, we can handle if there are any mismatches
> by using the safe value. e.g, HWCAPs and other features used by the
> kernel. Relax the constraint on the feature bits whose mismatch can
> be handled by the kernel.
> 
> For VHE, if there is a mismatch we don't care if the kernel is
> not using it. If the kernel is indeed running in EL2 mode, then
> the mismatches results in a panic. Similarly for ASID bits we
> take care of conflicts.
> 
> For other features like, PAN, UAO we only enable it only if we
> have it on all the CPUs. For IESB, we set the SCTLR bit unconditionally
> anyways.
> 
> For features that aren't currently used by kernel
>  (e.g ID_AA64MFMR1:{LOR,HPD}, ID_AA64MMFR2:LSM) make them NONSTRICT.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: James Morse <james.morse@arm.com>
> Cc: Dave Martin <dave.martin@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> Changes since v1:
>  - Make ID_AA64MMFR1_EL1:LOR/HPD, ID_AA64MMFR1_EL1:LSM non-strict
>    as they aren't used by the kernel.
>  - Added comments around different fields.
>  - Make ID_AA64MMFR2:CNP non-strict, as we could decide to use it
>    only when it is available on all the CPUs.

This does mean we need to be careful when adding support for a new feature
because the cpufeature code is no longer guaranteeing homogeneity. I can't
see how we can detect this, so I suppose we'll just need to be careful to
pick this up during review.

It's also a bit nasty that older kernels won't shout about mismatched
features but a new kernel might. I have a slight concern that this means
integration problems might slip through the cracks when a design is
validating against an older kernel.

Finally, there's still the problem that some features cannot be
enabled/disabled by the kernel and we can end up in a position where a
user application might SIGILL only on some CPUs if it's using an instruction
that isn't supported across the whole system. I think that sort of
configuration *does* warrant the current sanity check message/taint; afaict
we still go ahead and use the safe value, clobbering things like the hwcap,
but we should draw attention to the fact that userspace might crash if it's
trying to probe for these instructions using traps.

I'd like to hear what others think about this. As it stands, I don't think
this patch is quite right but I wouldn't be against relaxing specific
features to be NONSTRICT where we know that the kernel today can deal with
that transparently to userspace.

Will