On 1/16/20 9:54 PM, Catalin Marinas wrote:

On Mon, Dec 16, 2019 at 02:17:08PM +0530, Amit Daniel Kachhap wrote:

quoted

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 5aaf1bb..c59c28f 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S

@@ -13,6 +13,7 @@
  #include <linux/init.h>
  #include <linux/irqchip/arm-gic-v3.h>
  
+#include <asm/alternative.h>
  #include <asm/assembler.h>
  #include <asm/boot.h>
  #include <asm/ptrace.h>

@@ -713,6 +714,7 @@ secondary_startup:
  	 * Common entry point for secondary CPUs.
  	 */
  	bl	__cpu_secondary_check52bitva
+	bl	__cpu_secondary_checkptrauth
  	mov	x0, #ARM64_CPU_BOOT_LATE
  	bl	__cpu_setup			// initialise processor
  	adrp	x1, swapper_pg_dir

@@ -831,6 +833,24 @@ __no_granule_support:
  	early_park_cpu CPU_STUCK_REASON_NO_GRAN
  ENDPROC(__no_granule_support)
  
+ENTRY(__cpu_secondary_checkptrauth)
+#ifdef CONFIG_ARM64_PTR_AUTH
+	/* Check if the CPU supports ptrauth */
+	mrs	x2, id_aa64isar1_el1
+	ubfx	x2, x2, #ID_AA64ISAR1_APA_SHIFT, #8
+	cbnz	x2, 1f
+alternative_if ARM64_HAS_ADDRESS_AUTH
+	mov	x3, 1
+alternative_else
+	mov	x3, 0
+alternative_endif
+	cbz	x3, 1f
+	/* Park the mismatched secondary CPU */
+	early_park_cpu CPU_STUCK_REASON_NO_PTRAUTH
+#endif
+1:	ret
+ENDPROC(__cpu_secondary_checkptrauth)

Do we actually need to park secondary CPUs early? Let's say a secondary
CPU doesn't have PAC, __cpu_setup won't set the corresponding SCTLR_EL1
bits and the instructions are NOPs. Wouldn't the cpufeature framework
park it later anyway?

In the current cpufeature framework, such missing cpufeature in
secondary cpu will lead to kernel panic (inside check_early_cpufeatures)
and not cpu offline. However Kristina in her RFC V2 [1] added such
feature to park it.

Later for moving the enabling ptrauth to assembly this work got dropped.
Suzuki provided the template code for doing that [2].

Later James suggested to do this like existing
__cpu_secondary_check52bitva which parks the secondary cpu very early
and also to save wasted cpu cycles [3].

So your question is still valid that it can be done in cpufeature. Let
me know your opinion that which one is better.

[1]: 
https://lore.kernel.org/linux-arm-kernel/20190529190332.29753-4-kristina.martsenko@arm.com/ (local)
[2]: 
https://lore.kernel.org/linux-arm-kernel/9886324a-5a12-5dd8-b84c-3f32098e3d35@arm.com/ (local)
[3]: https://www.spinics.net/lists/arm-kernel/msg763622.html

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