Messages in this thread |  | | Subject | Re: [PATCH v5 03/27] arm64: alternative: Apply alternatives early in boot process | From | Julien Thierry <> | Date | Wed, 12 Sep 2018 17:49:09 +0100 |
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Hi James,
On 12/09/18 11:29, James Morse wrote: > Hi Julien, > > On 28/08/18 16:51, Julien Thierry wrote: >> From: Daniel Thompson <daniel.thompson@linaro.org> >> >> Currently alternatives are applied very late in the boot process (and >> a long time after we enable scheduling). Some alternative sequences, >> such as those that alter the way CPU context is stored, must be applied >> much earlier in the boot sequence. >> >> Introduce apply_boot_alternatives() to allow some alternatives to be >> applied immediately after we detect the CPU features of the boot CPU. > > >> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c >> index b5d6039..70c2604 100644 >> --- a/arch/arm64/kernel/alternative.c >> +++ b/arch/arm64/kernel/alternative.c >> @@ -145,7 +145,8 @@ static void clean_dcache_range_nopatch(u64 start, u64 end) >> } while (cur += d_size, cur < end); >> } >> >> -static void __apply_alternatives(void *alt_region, bool is_module) >> +static void __apply_alternatives(void *alt_region, bool is_module, >> + unsigned long feature_mask) > > Shouldn't feature_mask be a DECLARE_BITMAP() maybe-array like cpu_hwcaps? > This means it keeps working when NR_CAPS grows over 64, which might happen > sooner than we think for backported errata... > > >> @@ -155,6 +156,9 @@ static void __apply_alternatives(void *alt_region, bool is_module) >> for (alt = region->begin; alt < region->end; alt++) { >> int nr_inst; >> >> + if ((BIT(alt->cpufeature) & feature_mask) == 0) >> + continue; >> + >> /* Use ARM64_CB_PATCH as an unconditional patch */ >> if (alt->cpufeature < ARM64_CB_PATCH && >> !cpus_have_cap(alt->cpufeature)) >> @@ -213,7 +217,7 @@ static int __apply_alternatives_multi_stop(void *unused) >> isb(); >> } else { >> BUG_ON(alternatives_applied); >> - __apply_alternatives(®ion, false); >> + __apply_alternatives(®ion, false, ~boot_capabilities); > > Ah, this is tricky. There is a bitmap_complement() for the DECLARE_BITMAP() > stuff, but we'd need a second array... > > We could pass the scope around, but then __apply_alternatives() would need to > lookup the struct arm64_cpu_capabilities up every time. This is only a problem > as we have one cap-number-space for errata/features, but separate sparse lists. >
Since for each alternative we know the cpufeature associated with it, the "lookup" is really just accessing an array with the given index, so that could be an option.
> (I think applying the alternatives one cap at a time is a bad idea as we would > need to walk the alternative region NR_CAPS times) > > >> @@ -227,6 +231,24 @@ void __init apply_alternatives_all(void) >> stop_machine(__apply_alternatives_multi_stop, NULL, cpu_online_mask); >> } >> >> +/* >> + * This is called very early in the boot process (directly after we run >> + * a feature detect on the boot CPU). No need to worry about other CPUs >> + * here. >> + */ >> +void __init apply_boot_alternatives(void) >> +{ >> + struct alt_region region = { >> + .begin = (struct alt_instr *)__alt_instructions, >> + .end = (struct alt_instr *)__alt_instructions_end, >> + }; >> + >> + /* If called on non-boot cpu things could go wrong */ >> + WARN_ON(smp_processor_id() != 0); > > Isn't the problem if there are multiple CPUs online? >
Yes, that makes more sense. I'll change this.
> >> + __apply_alternatives(®ion, false, boot_capabilities); >> +} >> + >> #ifdef CONFIG_MODULES >> void apply_alternatives_module(void *start, size_t length) >> { > >> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >> index 3bc1c8b..0d1e41e 100644 >> --- a/arch/arm64/kernel/cpufeature.c >> +++ b/arch/arm64/kernel/cpufeature.c >> @@ -52,6 +52,8 @@ >> DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS); >> EXPORT_SYMBOL(cpu_hwcaps); >> >> +unsigned long boot_capabilities; >> + >> /* >> * Flag to indicate if we have computed the system wide >> * capabilities based on the boot time active CPUs. This >> @@ -1375,6 +1377,9 @@ static void __update_cpu_capabilities(const struct arm64_cpu_capabilities *caps, >> if (!cpus_have_cap(caps->capability) && caps->desc) >> pr_info("%s %s\n", info, caps->desc); >> cpus_set_cap(caps->capability); > > Hmm, the bitmap behind cpus_set_cap() is what cpus_have_cap() in > __apply_alternatives() looks at. If you had a call to __apply_alternatives after > update_cpu_capabilities(SCOPE_BOOT_CPU), but before any others, it would only > apply those alternatives... > > (I don't think there is a problem re-applying the same alternative, but I > haven't checked). >
Interesting idea. If someone can confirm that patching alternatives twice is safe, I think it would make things simpler.
Thanks,
-- Julien Thierry
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