Re: [PATCH v2 15/15] efi/arm64: Implement ARM64 DRTM in the stub

[Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>]

On Fri,  2 Oct 2026 20:31:52 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> Provide an implementation of the CONFIG_EFI_STUB_DRTM protocol for ARM64
> DEN0113 >= v1.1.
> 
> First, it queries the FW for support and collects all the
> information. This is needed to compute the extra_size, which comes
> from FW reporting how much memory it needs during the launch for the
> DLME Data and DCE-owned data. The generic stub ensures there is
> trailing memory after Image for this.
> 
> Then the DRTM_PARAMETERS launch structure is computed using the
> offsets in the efi_info.
> 
> Finally, the stub does ExitBootServices and calls the actual launch.
> 
> The feature discovery process is deliberately fairly verbose to help
> debug any FW weirdness in the field. Enable it with efi=debug
> 
> It looks something like:
> 
>  EFI stub: DRTM: interface version 1.4
>  EFI stub: DEBUG: DRTM: TPM algorithm 0xc, TPM hashing unavailable, PCR schemas 0x1
>  EFI stub: DEBUG: DRTM: minimum DLME data 73728 bytes, Normal-world DCE 0 bytes
>  EFI stub: Decompressing Linux Kernel...
>  EFI stub: Generating empty DTB
>  EFI stub: Exiting boot services...
>  EFI stub: DEBUG: DRTM: will launch, selected launch features 0x0
>  [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x000f0510]
> 
> Eventually the launch'd kernel is going to require built in crypto
> libraries that match what the FW is using so it can validate some of the
> information left behind during early boot. Refuse to launch if these are
> not built in and build in the most common ones from the ARM64_DRTM
> kconfig.
> 
> One nit, if the platform does not support SMC then using drtm=auto may
> crash on the SMC op. As far as I can tell there is no way to discover SMC
> support through EFI. Learning it from the ACPI FADT is doable and costs
> about 250 lines of code.

Ouch. Sounds like a must have if there is no easier way.

> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

One thing inline that confused me.

Otherwise seems fine to me

> diff --git a/drivers/firmware/efi/libstub/arm64-drtm.c b/drivers/firmware/efi/libstub/arm64-drtm.c
> new file mode 100644
> index 00000000000000..2fc58babc3e7cb
> --- /dev/null
> +++ b/drivers/firmware/efi/libstub/arm64-drtm.c
> @@ -0,0 +1,426 @@

> +
> +/* True if SMCCC 1.0 or later is available. */
> +static bool efi_arm64_psci_smccc_compatible(void)
> +{
> +	struct arm_smccc_res res;
> +	s32 status;
> +
> +	arm_smccc_1_1_smc(PSCI_0_2_FN_PSCI_VERSION, &res);

A function that is meant to check for smccc 1.0 is using the
smccc_1_1 namespaced calls?  Maybe add a comment on why that
is ok to do.  Is anyone actually using SMCCC 1.0 on systems like this?

I a bit surprised we don't have a common helper for this -
I guess the version checks are often explicit in a particular
driver being bound.

> +	if ((s32)res.a0 < 0)
> +		return false;
> +
> +	if (PSCI_VERSION_MAJOR(res.a0) < 1)
> +		return false;
> +
> +	arm_smccc_1_1_smc(PSCI_1_0_FN_PSCI_FEATURES, ARM_SMCCC_VERSION_FUNC_ID,
> +			  &res);
> +	status = (s32)res.a0;
> +	if (status == PSCI_RET_NOT_SUPPORTED) {
> +		/*
> +		 * SMCCC_VERSION was introduced by SMCCC 1.1, so firmware that
> +		 * does not advertise that function through PSCI_FEATURES is
> +		 * using the valid SMCCC 1.0 baseline.
> +		 */
> +		return true;
> +	} else if (status < 0) {
> +		return false;
> +	} else {
> +		u32 version;
> +
> +		arm_smccc_1_1_smc(ARM_SMCCC_VERSION_FUNC_ID, &res);
> +		version = res.a0;
> +		if ((version >> 16) != 1 || version < ARM_SMCCC_VERSION_1_1)
> +			return false;
> +	}
> +	return true;
> +}