Re: [PATCH 6/9] arm64: alternatives: have callbacks take a cap

Joey Gouly
2022-09-02T16:11:32Z

Hi Mark,

On Thu, Sep 01, 2022 at 04:14:00PM +0100, Mark Rutland wrote:
> Today, callback alternatives are special-cased within
> __apply_alternatives(), and are applied alongside patching for system
> capabilities as ARM64_NCAPS is not part of the boot_capabilities feature
> mask.
>
> This special-casing is less than ideal. Giving special meaning to
> ARM64_NCAPS for this requires some structures and loops to use
> ARM64_NCAPS + 1 (AKA ARM64_NPATCHABLE), while others use ARM64_NCAPS.
> It's also not immediately clear callback alternatives are only applied
> when applying alternatives for system-wide features.
>
> To make this a bit clearer, changes the way that callback alternatives
> are identified to remove the special-casing of ARM64_NCAPS, and to allow
> callback alternatives to be associated with a cpucap as with all other
> alternatives.
>
> New cpucaps, ARM64_ALWAYS_BOOT and ARM64_ALWAYS_SYSTEM are added which
> are always detected alongside boot cpu capabilities and system
> capabilities respectively. All existing callback alternatives are made
> to use ARM64_ALWAYS_SYSTEM, and so will be patched at the same point
> during the boot flow as before.
>
> Subsequent patches will make more use of these new cpucaps.
>
> There should be no functional change as a result of this patch.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: James Morse <james.morse@arm.com>
> Cc: Joey Gouly <joey.gouly@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> ---
> arch/arm64/include/asm/alternative-macros.h | 18 ++++++++-----
> arch/arm64/include/asm/assembler.h | 10 ++++----
> arch/arm64/include/asm/cpufeature.h | 4 +--
> arch/arm64/include/asm/kvm_mmu.h | 5 ++--
> arch/arm64/kernel/alternative.c | 28 ++++++++++++---------
> arch/arm64/kernel/cpufeature.c | 19 ++++++++++++--
> arch/arm64/kernel/entry.S | 8 +++---
> arch/arm64/kvm/hyp/hyp-entry.S | 4 +--
> arch/arm64/tools/cpucaps | 2 ++
> 9 files changed, 62 insertions(+), 36 deletions(-)
>

[..]

> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index 2e18c9c0f612b..da706c9f9a9a5 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -21,6 +21,11 @@
> #define ALT_ORIG_PTR(a) __ALT_PTR(a, orig_offset)
> #define ALT_REPL_PTR(a) __ALT_PTR(a, alt_offset)
>
> +#define ALT_CAP(a) ((a)->cpufeature & ~ARM64_CB_BIT)
> +#define ALT_HAS_CB(a) ((a)->cpufeature & ARM64_CB_BIT)
> +
> +#define ALT_NR_INST(a) ((a)->orig_len / AARCH64_INSN_SIZE)
You introduced this macro, but don't use it.

> +
> /* Volatile, as we may be patching the guts of READ_ONCE() */
> static volatile int all_alternatives_applied;
>
> @@ -143,16 +148,15 @@ static void __nocfi __apply_alternatives(const struct alt_region *region,
>
> for (alt = region->begin; alt < region->end; alt++) {
> int nr_inst;
> + int cap = ALT_CAP(alt);
>
> - if (!test_bit(alt->cpufeature, feature_mask))
> + if (!test_bit(cap, feature_mask))
> continue;
>
> - /* Use ARM64_CB_PATCH as an unconditional patch */
> - if (alt->cpufeature < ARM64_CB_PATCH &&
> - !cpus_have_cap(alt->cpufeature))
> + if (!cpus_have_cap(cap))
> continue;
>
> - if (alt->cpufeature == ARM64_CB_PATCH)
> + if (ALT_HAS_CB(alt))
> BUG_ON(alt->alt_len != 0);
> else
> BUG_ON(alt->alt_len != alt->orig_len);
> @@ -161,10 +165,10 @@ static void __nocfi __apply_alternatives(const struct alt_region *region,
> updptr = is_module ? origptr : lm_alias(origptr);
> nr_inst = alt->orig_len / AARCH64_INSN_SIZE;
>
> - if (alt->cpufeature < ARM64_CB_PATCH)
> - alt_cb = patch_alternative;
> - else
> + if (ALT_HAS_CB(alt))
> alt_cb = ALT_REPL_PTR(alt);
> + else
> + alt_cb = patch_alternative;
>
> alt_cb(alt, origptr, updptr, nr_inst);
>
> @@ -208,10 +212,10 @@ static int __apply_alternatives_multi_stop(void *unused)
> cpu_relax();
> isb();
> } else {
> - DECLARE_BITMAP(remaining_capabilities, ARM64_NPATCHABLE);
> + DECLARE_BITMAP(remaining_capabilities, ARM64_NCAPS);
>
> bitmap_complement(remaining_capabilities, boot_capabilities,
> - ARM64_NPATCHABLE);
> + ARM64_NCAPS);
>
> BUG_ON(all_alternatives_applied);
> __apply_alternatives(&kernel_alternatives, false,
> @@ -254,9 +258,9 @@ void apply_alternatives_module(void *start, size_t length)
> .begin = start,
> .end = start + length,
> };
> - DECLARE_BITMAP(all_capabilities, ARM64_NPATCHABLE);
> + DECLARE_BITMAP(all_capabilities, ARM64_NCAPS);
>
> - bitmap_fill(all_capabilities, ARM64_NPATCHABLE);
> + bitmap_fill(all_capabilities, ARM64_NCAPS);
>
> __apply_alternatives(&region, true, &all_capabilities[0]);
> }

Thanks,
Joey