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

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

On Wed, 4 Jan 2023 at 18:23, Ard Biesheuvel <ardb@kernel.org> 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                                        |  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

This is wrong - this should be '-mstrict-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
>