[PATCH] Fix 32-bit ARM handling of the CTR register

[Marc Zyngier <maz@kernel.org>]

When booting on an ARMv8 core that implements either CTR.IDC or CTR.DIC
(indicating that some of the cache maintenance operations can be
removed when dealing with I/D-cache coherency, GRUB dies with a
"Unsupported cache type 0x........" message.

This is pretty likely to happen when running in a virtual machine
hosted on an arm64 machine (I've triggered it on a system built around
a bunch of Cortex-A55 cores, which implements CTR.IDC).

It turns out that the way GRUB deals with the CTR register is a bit
harsh for anything from ARMv7 onwards. The layout of the register is
backward compatible, meaning that nothing that gets added is allowed to
break earlier behaviour. In this case, ignoring IDC is completely fine,
and only results in unnecessary cache maintenance.

We can thus avoid being paranoid, and align the 32bit behaviour with
its 64bit equivalent.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 grub-core/kern/arm/cache.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/grub-core/kern/arm/cache.c b/grub-core/kern/arm/cache.c
index af1c4bbf5..6c75193e4 100644
--- a/grub-core/kern/arm/cache.c
+++ b/grub-core/kern/arm/cache.c
@@ -93,13 +93,16 @@ probe_caches (void)
       grub_arch_cache_ilinesz = 8 << (cache_type & 3);
       type = ARCH_ARMV6;
       break;
-    case 0x80 ... 0x8f:
+    default:
+      /*
+       * The CTR register is pretty much unchanged from v7 onwards,
+       * and is guaranteed to be backward compatible (the IDC/DIC bits
+       * allow certain CMOs to be elided, but performing them is never
+       * wrong), hence handling it like its AArch64 equivalent.
+       */
       grub_arch_cache_dlinesz = 4 << ((cache_type >> 16) & 0xf);
       grub_arch_cache_ilinesz = 4 << (cache_type & 0xf);
       type = ARCH_ARMV7;
-      break;
-    default:
-      grub_fatal ("Unsupported cache type 0x%x", cache_type);
     }
   if (grub_arch_cache_dlinesz > grub_arch_cache_ilinesz)
     grub_arch_cache_max_linesz = grub_arch_cache_dlinesz;
-- 
2.26.2