Re: [PATCH v2 2/2] ArmVirtPkg/ArmVirtQemu: Avoid early ID map on ThunderX

["dann frazier" <dann.frazier@canonical.com>]

On Thu, Jan 05, 2023 at 05:25:28PM +0100, Ard Biesheuvel wrote:
> 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                                        |  5 +++++
>  ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S | 15 +++++++++++++++
>  2 files changed, 20 insertions(+)

FTR, this v2 series also worked for me.

  -dann

> diff --git a/ArmVirtPkg/ArmVirtQemu.dsc b/ArmVirtPkg/ArmVirtQemu.dsc
> index f77443229e8e..5dd8b6104cca 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 +118,11 @@ [LibraryClasses.common.UEFI_DRIVER]
>    UefiScsiLib|MdePkg/Library/UefiScsiLib/UefiScsiLib.inf
>  
>  [BuildOptions]
> +!if $(CAVIUM_ERRATUM_27456) == TRUE
> +  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 1787d52fbf51..5ac7c732f6ec 100644
> --- a/ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S
> +++ b/ArmVirtPkg/Library/ArmPlatformLibQemu/AArch64/ArmPlatformHelper.S
> @@ -42,6 +42,21 @@
>  
>  
>  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 (but not 0xAF)
> +   */
> +  mrs    x0, midr_el1            // read the MIDR into X0
> +  ubfx   x1, x0, #24, #8         // grab implementor id
> +  ubfx   x0, x0, #7, #9          // grab part number bits [11:3]
> +  cmp    x1, #0x43               // compare implementor id
> +  ccmp   x0, #0xA0 >> 3, #0, eq  // compare part# bits [11:3]
> +  b.eq   0f
> +#endif
>    mrs    x0, CurrentEL           // check current exception level
>    tbnz   x0, #3, 0f              // omit early ID map if above EL1
>