[PATCH] ArmVirtPkg/ArmVirtQemu: Avoid early ID map on ThunderX

["Ard Biesheuvel" <ardb@kernel.org>]

The early ID map used by ArmVirtQemu uses ASID scoped non-global
mappings, as this allows us to switch to the permanent ID map seamlessly
without the need for explicit TLB maintenance.

However, this triggers a known erratum on ThunderX, which does not
tolerate non-global mappings that are executable at EL1, as this appears
to result in I-cache corruption. (Linux disables the KPTI based Meltdown
mitigation on ThunderX for the same reason)

So work around this, by detecting the CPU implementor and part number,
and proceeding without the early ID map if a ThunderX CPU is detected.

Note that this requires the C code to be built with strict alignment
again, as we may end up executing it with the MMU and caches off.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 ArmVirtPkg/ArmVirtQemu.dsc                                        |  6 ++++++
 ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S | 18 ++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/ArmVirtPkg/ArmVirtQemu.dsc b/ArmVirtPkg/ArmVirtQemu.dsc
index f77443229e8e..340b36f69c2c 100644
--- a/ArmVirtPkg/ArmVirtQemu.dsc
+++ b/ArmVirtPkg/ArmVirtQemu.dsc
@@ -31,6 +31,7 @@ [Defines]
   DEFINE SECURE_BOOT_ENABLE      = FALSE
   DEFINE TPM2_ENABLE             = FALSE
   DEFINE TPM2_CONFIG_ENABLE      = FALSE
+  DEFINE CAVIUM_ERRATUM_27456    = FALSE
 
   #
   # Network definition
@@ -117,7 +118,12 @@ [LibraryClasses.common.UEFI_DRIVER]
   UefiScsiLib|MdePkg/Library/UefiScsiLib/UefiScsiLib.inf
 
 [BuildOptions]
+!if $(CAVIUM_ERRATUM_27456) == TRUE
+  GCC:*_*_AARCH64_CC_XIPFLAGS = -mno-strict-align
+  GCC:*_*_AARCH64_PP_FLAGS    = -DCAVIUM_ERRATUM_27456
+!else
   GCC:*_*_AARCH64_CC_XIPFLAGS ==
+!endif
 
 !include NetworkPkg/NetworkBuildOptions.dsc.inc
 
diff --git a/ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S b/ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S
index 05ccc7f9f043..962f1ba3a4d7 100644
--- a/ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S
+++ b/ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S
@@ -44,8 +44,26 @@
 
 
 ASM_FUNC(ArmPlatformPeiBootAction)
+#ifdef CAVIUM_ERRATUM_27456
+  /*
+   * On Cavium ThunderX, using non-global mappings that are executable at EL1
+   * results in I-cache corruption. So just avoid the early ID mapping there.
+   *
+   * MIDR implementor   0x43
+   * MIDR part numbers  0xA1 0xA2
+   */
+  mrs    x0, midr_el1            // read the MIDR into X0
+  ubfx   x1, x0, #6, #10         // grab part number bits [11:2]
+  ubfx   x0, x0, #24, #8         // grab implementor id
+  mov    x2, #0xA0 >> 2
+  cmp    x0, #0x43               // compare implementor id
+  ccmp   x1, x2, #0, eq          // compare part# bits [11:2]
+  b.eq   .Lreturn
+#endif
+
   mrs    x0, CurrentEL           // check current exception level
   tbz    x0, #3, 0f              // bail if above EL1
+.Lreturn:
   ret
 
 0:mov_i  x0, mairval
-- 
2.39.0