arm64: head: move memstart_offset_seed handling to C code

Now that we can set BSS variables from the early code running from the
ID map, we can set memstart_offset_seed directly from the C code that
derives the value instead of passing it back and forth between C and asm
code.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-60-ardb+git@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/head.S

@@ -82,7 +82,6 @@
 	 *  x21        primary_entry() .. start_kernel()        FDT pointer passed at boot in x0
 	 *  x22        create_idmap() .. start_kernel()         ID map VA of the DT blob
 	 *  x23        __primary_switch()                       physical misalignment/KASLR offset
-	 *  x24        __primary_switch()                       linear map KASLR seed
 	 *  x25        primary_entry() .. start_kernel()        supported VA size
 	 *  x28        create_idmap()                           callee preserved temp register
 	 */
@@ -483,11 +482,6 @@ SYM_FUNC_START_LOCAL(__primary_switched)
 	str	x25, [x8]			// ... observes the correct value
 	dc	civac, x8			// Make visible to booting secondaries
 #endif
-
-#ifdef CONFIG_RANDOMIZE_BASE
-	adrp	x5, memstart_offset_seed	// Save KASLR linear map seed
-	strh	w24, [x5, :lo12:memstart_offset_seed]
-#endif
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 	bl	kasan_early_init
 #endif
@@ -779,7 +773,6 @@ SYM_FUNC_START_LOCAL(__primary_switch)
 #ifdef CONFIG_RANDOMIZE_BASE
 	mov	x0, x22
 	bl	__pi_kaslr_early_init
-	and	x24, x0, #SZ_2M - 1		// capture memstart offset seed
 	bic	x0, x0, #SZ_2M - 1
 	orr	x23, x23, x0			// record kernel offset
 #endif

arch/arm64/kernel/image-vars.h

@@ -46,6 +46,7 @@ PROVIDE(__pi_id_aa64smfr0_override	= id_aa64smfr0_override);
 PROVIDE(__pi_id_aa64zfr0_override	= id_aa64zfr0_override);
 PROVIDE(__pi_arm64_sw_feature_override	= arm64_sw_feature_override);
 PROVIDE(__pi__ctype			= _ctype);
+PROVIDE(__pi_memstart_offset_seed	= memstart_offset_seed);
 
 #ifdef CONFIG_KVM
 

arch/arm64/kernel/pi/kaslr_early.c

@@ -16,6 +16,8 @@
 #include <asm/memory.h>
 #include <asm/pgtable.h>
 
+extern u16 memstart_offset_seed;
+
 static u64 __init get_kaslr_seed(void *fdt)
 {
 	static char const chosen_str[] __initconst = "chosen";
@@ -51,6 +53,8 @@ asmlinkage u64 __init kaslr_early_init(void *fdt)
 			return 0;
 	}
 
+	memstart_offset_seed = seed & U16_MAX;
+
 	/*
 	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 	 * kernel image offset from the seed. Let's place the kernel in the