[PATCH 09/37] arm64: kvm: Use cpus_have_final_cap() explicitly

[Marc Zyngier]

On Tue, 19 Sep 2023 10:28:22 +0100,
Mark Rutland <mark.rutland at arm.com> wrote:
> 
> Much of the arm64 KVM code uses cpus_have_const_cap() to check for
> cpucaps, but this is unnecessary and it would be preferable to use
> cpus_have_final_cap().
> 
> For historical reasons, cpus_have_const_cap() is more complicated than
> it needs to be. Before cpucaps are finalized, it will perform a bitmap
> test of the system_cpucaps bitmap, and once cpucaps are finalized it
> will use an alternative branch. This used to be necessary to handle some
> race conditions in the window between cpucap detection and the
> subsequent patching of alternatives and static branches, where different
> branches could be out-of-sync with one another (or w.r.t. alternative
> sequences). Now that we use alternative branches instead of static
> branches, these are all patched atomically w.r.t. one another, and there
> are only a handful of cases that need special care in the window between
> cpucap detection and alternative patching.
> 
> Due to the above, it would be nice to remove cpus_have_const_cap(), and
> migrate callers over to alternative_has_cap_*(), cpus_have_final_cap(),
> or cpus_have_cap() depending on when their requirements. This will
> remove redundant instructions and improve code generation, and will make
> it easier to determine how each callsite will behave before, during, and
> after alternative patching.
> 
> KVM is initialized after cpucaps have been finalized and alternatives
> have been patched. Since commit:
> 
>   d86de40decaa14e6 ("arm64: cpufeature: upgrade hyp caps to final")
> 
> ... use of cpus_have_const_cap() in hyp code is automatically converted
> to use cpus_have_final_cap():
> 
> | static __always_inline bool cpus_have_const_cap(int num)
> | {
> | 	if (is_hyp_code())
> | 		return cpus_have_final_cap(num);
> | 	else if (system_capabilities_finalized())
> | 		return __cpus_have_const_cap(num);
> | 	else
> | 		return cpus_have_cap(num);
> | }
> 
> Thus, converting hyp code to use cpus_have_final_cap() directly will not
> result in any functional change.
> 
> Non-hyp KVM code is also not executed until cpucaps have been finalized,
> and it would be preferable to extent the same treatment to this code and
> use cpus_have_final_cap() directly.
> 
> This patch converts instances of cpus_have_const_cap() in KVM-only code
> over to cpus_have_final_cap(). As all of this code runs after cpucaps
> have been finalized, there should be no functional change as a result of
> this patch, but the redundant instructions generated by
> cpus_have_const_cap() will be removed from the non-hyp KVM code.

Reviewed-by: Marc Zyngier <maz at kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.