On 10/02/2020 12:27, Mark Rutland wrote:
When cpus_have_const_cap() was originally introduced it was intended to
be safe in hyp context, where it is not safe to access the cpu_hwcaps
array as cpus_have_cap() did. For more details see commit:
a4023f682739439b ("arm64: Add hypervisor safe helper for checking constant capabilities")
We then made use of cpus_have_const_cap() throughout the kernel.
Subsequently, we had to defer updating the static_key associated with
each capability in order to avoid lockdep complaints. To avoid breaking
kernel-wide usage of cpus_have_const_cap(), this was updated to fall
back to the cpu_hwcaps array if called before the static_keys were
updated. As the kvm hyp code was only called later than this, the
fallback is redundant but not functionally harmful. For more details,
see commit:
63a1e1c95e60e798 ("arm64/cpufeature: don't use mutex in bringup path")
Today we have more users of cpus_have_const_cap() which are only called
once the relevant static keys are initialized, and it would be
beneficial to avoid the redundant code.
To that end, this patch adds a new cpus_have_final_cap(), helper which
is intend to be used in code which is only run once capabilities have
been finalized, and will never check the cpus_hwcap array. This helps
the compiler to generate better code as it no longer needs to generate
code to address and test the cpus_hwcap array. To help catch misuse,
cpus_have_final_cap() will BUG() if called before capabilities are
finalized.
In hyp context, BUG() will result in a hyp panic, but the specific BUG()
instance will not be identified in the usual way.
Comments are added to the various cpus_have_*_cap() helpers to describe
the constraints on when they can be used. For clarity cpus_have_cap() is
moved above the other helpers.
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Suzuki Poulose <suzuki.poulose@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
---
...
+/*
+ * Test for a capability without a runtime check.
+ *
+ * Before capabilities are finalized, this will BUG().
+ * After capabilities are finalized, this is patched to avoid a runtime check.
+ *
+ * @num must be a compile-time constant.
+ */
+static __always_inline bool cpus_have_final_cap(int num)
+{
+ if (static_branch_likely(&arm64_const_caps_ready))
We have introduced system_capabilities_finalized() helper and may be
it is a good idea to use it here, to make it more clear.
Either ways :
Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
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