On Fri, May 04, 2018 at 11:06:56AM +0100, 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:

quoted

From: Daniel Thompson <redacted>

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_alternatives_early() to allow some alternatives to be
applied immediately after we detect the CPU features of the boot CPU.

Signed-off-by: Daniel Thompson <redacted>
Signed-off-by: Julien Thierry <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
---
  arch/arm64/include/asm/alternative.h |  1 +
  arch/arm64/kernel/alternative.c      | 39 +++++++++++++++++++++++++++++++++---
  arch/arm64/kernel/smp.c              |  6 ++++++
  3 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index 4a85c69..1fc1cdb 100644
--- a/arch/arm64/include/asm/alternative.h
+++ b/arch/arm64/include/asm/alternative.h

@@ -20,6 +20,7 @@ struct alt_instr {
  	u8  alt_len;		/* size of new instruction(s), <= orig_len */
  };

+void __init apply_alternatives_early(void);
  void __init apply_alternatives_all(void);
  void apply_alternatives(void *start, size_t length);

diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 6dd0a3a3..78051d4 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c

@@ -28,6 +28,18 @@
  #include <asm/sections.h>
  #include <linux/stop_machine.h>

+/*
+ * 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.

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.

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).

Any thoughts or preferences on this?

If I understand ARM64_CPUCAP_SCOPE_BOOT_CPU correctly it certainly seems
safe to apply the alternatives early (it means that a CPU that 
contradicts a CSCOPE_BOOT_CPU won't be allowed to join the system,
right?).

It also makes the system to apply errata fixes more powerful: maybe a
future errata must be applied before we commence threading.

This I have a preference for striping this out and relying on
SCOPE_BOOT_CPU instead. It's a weak preference though since I haven't
studied exactly what errate fixes this will bring into the scope of
early boot.

I don't think you'll regret changing it. This patch has always been
a *total* PITA to rebase so aligning it better with upstream will make
it easier to nurse the patch set until the if-and-when point it hits
the upstream.


Daniel.

Thanks,

quoted

  #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

-- 
Julien Thierry