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

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

On Wed, 21 Oct 2026 20:39:09 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Wed, Oct 21, 2026 at 11:11:15AM -0700, Jonathan Cameron wrote:
> > 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.  
> 
> Yeah, maybe so. I will include the patch in the next version. Though
> I'm not keen on it and I don't know very much about ACPI.
> 
> > > +/* 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?  
> 
> This is a good point, I didn't quite get all these details at the
> time, but yeah, technically we have to use the 1.0 calling convention
> due the register clobber differnces. The other reason is we can't use
> the kernels 1.0 helper since it is an asm function not an assembly..
> 
> Fixing everything I came with this:
> 
> /*
>  * Discovery must tolerate SMCCC 1.0 clobbering x0-x17. The EFI stub cannot
>  * call the kernel's out-of-line SMCCC helper, so issue these calls inline.
>  */
> static u32 efi_arm64_smc_1_0(u32 function, u32 arg)
> {
> 	register unsigned long x0 asm("x0") = function;
> 	register unsigned long x1 asm("x1") = arg;
> 
> 	asm volatile("smc #0"
> 		     : "+r" (x0), "+r" (x1)
> 		     :
> 		     : "x2", "x3", "x4", "x5", "x6", "x7", "x8", "x9",
> 		       "x10", "x11", "x12", "x13", "x14", "x15", "x16",
> 		       "x17", "cc", "memory");
> 
> 	return x0;
> }
> 
> /* True if SMCCC 1.1 or later in the 1.x series is available. */
> static bool efi_arm64_psci_smccc_compatible(void)
> {
> 	u32 version;
> 	s32 status;
> 
> 	version = efi_arm64_smc_1_0(PSCI_0_2_FN_PSCI_VERSION, 0);
> 	if ((s32)version < 0)
> 		return false;
> 
> 	if (PSCI_VERSION_MAJOR(version) < 1)
> 		return false;
> 
> 	status = efi_arm64_smc_1_0(PSCI_1_0_FN_PSCI_FEATURES,
> 				   ARM_SMCCC_VERSION_FUNC_ID);
> 	if (status < 0)
> 		return false;
> 
> 	version = efi_arm64_smc_1_0(ARM_SMCCC_VERSION_FUNC_ID, 0);
> 	return (version >> 16) == 1 && version >= ARM_SMCCC_VERSION_1_1;
> }
> 
> efistub can get away with this inline asm since it isn't 32 bit and it
> doesn't deal with quirks.

Smells plausible to me.

> 
> > 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.  
> 
> The PSCI stuff in the kernel is all very early when establishing what
> the channel is and is done only once. Here is a bit different
> since we only accept the smc instruction used by drtm..
> 
> And even if there was a helper we can't really call it from zboot..

True enough. I was just hoping there was code to compare this with :)

> 
> Thanks,
> Jason