Re: [PATCH v2 2/6] arm64: alternative: Apply alternatives early in boot process

[Suzuki K Poulose]

On 05/04/2018 11:06 AM, Julien Thierry wrote:
Hi,
In order to prepare the v3 of this patchset, I'd like people's opinion 
on what this patch does. More below.

On 17/01/18 11:54, Julien Thierry wrote:
From: Daniel Thompson <daniel.thomp...@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.

+/*
+ * early-apply features are detected using only the boot CPU and 
checked on
+ * secondary CPUs startup, even then,
+ * These early-apply features should only include features where we must
+ * patch the kernel very early in the boot process.
+ *
+ * Note that the cpufeature logic *must* be made aware of early-apply
+ * features to ensure they are reported as enabled without waiting
+ * for other CPUs to boot.
+ */
+#define EARLY_APPLY_FEATURE_MASK BIT(ARM64_HAS_SYSREG_GIC_CPUIF)
+

Following the change in the cpufeature infrastructure, 
ARM64_HAS_SYSREG_GIC_CPUIF will have the scope 
ARM64_CPUCAP_SCOPE_BOOT_CPU in order to be checked early in the boot 
process.

Thats correct.


Now, regarding the early application of alternative, I am wondering 
whether we can apply all the alternatives associated with SCOPE_BOOT 
features that *do not* have a cpu_enable callback.


I don't understand why would you skip the ones that have a "cpu_enable" 
callback. Could you explain this a bit ? Ideally you should be able to
apply the alternatives for features with the SCOPE_BOOT, provided the
cpu_enable() callback is written properly.


Otherwise we can keep the macro to list individually each feature that 
is patchable at boot time as the current patch does (or put this info in 
a flag within the arm64_cpu_capabilities structure)

You may be able to build up the mask of *available* capabilities with 
SCOPE_BOOT at boot time by playing some trick in the 
setup_boot_cpu_capabilities(), rather than embedding it in the 
capabilities (and then parsing the entire table(s)) or manually keeping
track of the capabilities by having a separate mask.

Suzuki


Any thoughts or preferences on this?

Thanks,

  #define __ALT_PTR(a,f)        ((void *)&(a)->f + (a)->f)
  #define ALT_ORIG_PTR(a)        __ALT_PTR(a, orig_offset)
  #define ALT_REPL_PTR(a)        __ALT_PTR(a, alt_offset)
@@ -105,7 +117,8 @@ static u32 get_alt_insn(struct alt_instr *alt, 
__le32 *insnptr, __le32 *altinsnp
      return insn;
  }

-static void __apply_alternatives(void *alt_region, bool 
use_linear_alias)
+static void __apply_alternatives(void *alt_region,  bool 
use_linear_alias,
+                 unsigned long feature_mask)
  {
      struct alt_instr *alt;
      struct alt_region *region = alt_region;
@@ -115,6 +128,9 @@ static void __apply_alternatives(void *alt_region, 
bool use_linear_alias)
          u32 insn;
          int i, nr_inst;

+        if ((BIT(alt->cpufeature) & feature_mask) == 0)
+            continue;
+
          if (!cpus_have_cap(alt->cpufeature))
              continue;

@@ -138,6 +154,21 @@ static void __apply_alternatives(void 
*alt_region, bool use_linear_alias)
  }

  /*
+ * 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 apply_alternatives_early(void)
+{
+    struct alt_region region = {
+        .begin    = (struct alt_instr *)__alt_instructions,
+        .end    = (struct alt_instr *)__alt_instructions_end,
+    };
+
+    __apply_alternatives(&region, true, EARLY_APPLY_FEATURE_MASK);
+}
+
+/*
   * We might be patching the stop_machine state machine, so implement a
   * really simple polling protocol here.
   */
@@ -156,7 +187,9 @@ static int __apply_alternatives_multi_stop(void 
*unused)
          isb();
      } else {
          BUG_ON(patched);
-        __apply_alternatives(&region, true);
+
+        __apply_alternatives(&region, true, ~EARLY_APPLY_FEATURE_MASK);
+
          /* Barriers provided by the cache flushing */
          WRITE_ONCE(patched, 1);
      }
@@ -177,5 +210,5 @@ void apply_alternatives(void *start, size_t length)
          .end    = start + length,
      };

-    __apply_alternatives(&region, false);
+    __apply_alternatives(&region, false, -1);
  }
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 551eb07..37361b5 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -453,6 +453,12 @@ void __init smp_prepare_boot_cpu(void)
       * cpuinfo_store_boot_cpu() above.
       */
      update_cpu_errata_workarounds();
+    /*
+     * We now know enough about the boot CPU to apply the
+     * alternatives that cannot wait until interrupt handling
+     * and/or scheduling is enabled.
+     */
+    apply_alternatives_early();
  }

  static u64 __init of_get_cpu_mpidr(struct device_node *dn)
--
1.9.1