arm64: alternatives: make clean_dcache_range_nopatch() noinstr-safe

arm64: alternatives: make clean_dcache_range_nopatch() noinstr-safe

When patching kernel alternatives, we need to be careful not to execute
kernel code which is itself subject to patching. In general, if code is
executed after the instructions in memory have been patched but prior to
the cache maintenance and barriers completing, it could lead to
UNPREDICTABLE results.

As our regular cache maintenance routines are patched with alternatives,
we have a clean_dcache_range_nopatch() function which is *intended* to
avoid patchable code and therefore supposed to be safe in the middle of
patching alternatives. Unfortunately, it's not marked as 'noinstr', and
so can be instrumented with patchable code.

Additionally, it calls read_sanitised_ftr_reg() (which may be
instrumented with patchable code) to find the sanitized value of
CTR_EL0.DminLine, and is therefore not safe to call during patching.

Luckily, since commit:

  675b0563d6b26aa9 ("arm64: cpufeature: expose arm64_ftr_reg struct for CTR_EL0")

... we can read the sanitised CTR_EL0 value directly, and avoid the call
to read_sanitised_ftr_reg().

This patch marks clean_dcache_range_nopatch() as noinstr, and has it
read the sanitized CTR_EL0 value directly, avoiding the issues above.

As a bonus, this is also an optimization. As read_sanitised_ftr_reg()
performs a binary search to find the CTR_EL0 value, reading the value
directly avoids this binary search per applied alternative, avoiding
some unnecessary work.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230616103150.1238132-1-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>