[PATCH v5 8/8] arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

According to the arm64 boot protocol, registers x1 to x3 should be
zero upon kernel entry, and non-zero values are reserved for future
use. This future use is going to be problematic if we never enforce
the current rules, so start enforcing them now, by emitting a warning
if non-zero values are detected.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/kernel/head.S  |  4 ++++
 arch/arm64/kernel/setup.c | 15 +++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index a0fbd99efb89..8636c3cef006 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -233,6 +233,10 @@ section_table:
 #endif
 
 ENTRY(stext)
+	adr_l	x8, boot_regs			// record the contents of
+	stp	x0, x1, [x8]			// x0 .. x3 at kernel entry
+	stp	x2, x3, [x8, #16]
+
 	mov	x21, x0				// x21=FDT
 	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
 	adrp	x24, __PHYS_OFFSET
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 6c5fb5aff325..2d5cae2de679 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -114,6 +114,11 @@ void __init early_print(const char *str, ...)
 	printk("%s", buf);
 }
 
+/*
+ * The recorded values of x0 .. x3 upon kernel entry.
+ */
+u64 __read_mostly boot_regs[4];
+
 void __init smp_setup_processor_id(void)
 {
 	u64 mpidr = read_cpuid_mpidr() & MPIDR_HWID_BITMASK;
@@ -387,6 +392,16 @@ void __init setup_arch(char **cmdline_p)
 	conswitchp = &dummy_con;
 #endif
 #endif
+	/*
+	 * boot_regs[] is written by the boot CPU with the caches off, so we
+	 * need to ensure that we read the value from main memory
+	 */
+	__flush_dcache_area(boot_regs, sizeof(boot_regs));
+	if (boot_regs[1] || boot_regs[2] || boot_regs[3]) {
+		pr_err("WARNING: boot protocol violation detected (x1 == %llx, x2 == %llx, x3 == %llx)\n",
+			boot_regs[1], boot_regs[2], boot_regs[3]);
+		pr_err("WARNING: your bootloader may fail to load newer kernels\n");
+	}
 }
 
 static int __init arm64_device_init(void)
-- 
1.8.3.2