arm64: Force KPTI to be disabled on Cavium ThunderX

Cavium ThunderX's erratum 27456 results in a corruption of icache
entries that are loaded from memory that is mapped as non-global
(i.e. ASID-tagged).

As KPTI is based on memory being mapped non-global, let's prevent
it from kicking in if this erratum is detected.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
[will: Update comment]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index cd9f469..0ff8ab7 100644 (file)
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -855,12 +855,23 @@ static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
 static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
                                int __unused)
 {
+       char const *str = "command line option";
        u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
 
-       /* Forced on command line? */
+       /*
+        * For reasons that aren't entirely clear, enabling KPTI on Cavium
+        * ThunderX leads to apparent I-cache corruption of kernel text, which
+        * ends as well as you might imagine. Don't even try.
+        */
+       if (cpus_have_const_cap(ARM64_WORKAROUND_CAVIUM_27456)) {
+               str = "ARM64_WORKAROUND_CAVIUM_27456";
+               __kpti_forced = -1;
+       }
+
+       /* Forced? */
        if (__kpti_forced) {
-               pr_info_once("kernel page table isolation forced %s by command line option\n",
-                            __kpti_forced > 0 ? "ON" : "OFF");
+               pr_info_once("kernel page table isolation forced %s by %s\n",
+                            __kpti_forced > 0 ? "ON" : "OFF", str);
                return __kpti_forced > 0;
        }