Subject: Re: [PATCH 1/2] powerpc/prom_init: Replace linux,sml-base/sml-size with linux,sml-log

From: Michael Ellerman <[email-masked]>
Date: Thu, 07 Mar 2024 21:41:31 +1100

Stefan Berger <[email-masked]> writes:
> linux,sml-base holds the address of a buffer with the TPM log. This
> buffer may become invalid after a kexec and therefore embed the whole TPM
> log in linux,sml-log. This helps to protect the log since it is properly
> carried across a kexec with both of the kexec syscalls.
>
> Signed-off-by: Stefan Berger <[email-masked]>
> ---
>  arch/powerpc/kernel/prom_init.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/kernel/prom_init.c b/arch/powerpc/kernel/prom_init.c
> index e67effdba85c..41268c30de4c 100644
> --- a/arch/powerpc/kernel/prom_init.c
> +++ b/arch/powerpc/kernel/prom_init.c
> @@ -1956,12 +1956,8 @@ static void __init prom_instantiate_sml(void)
>  
>  	reserve_mem(base, size);
>  
> -	prom_setprop(ibmvtpm_node, "/vdevice/vtpm", "linux,sml-base",
> -		     &base, sizeof(base));
> -	prom_setprop(ibmvtpm_node, "/vdevice/vtpm", "linux,sml-size",
> -		     &size, sizeof(size));
> -
> -	prom_debug("sml base     = 0x%llx\n", base);
> +	prom_setprop(ibmvtpm_node, "/vdevice/vtpm", "linux,sml-log",
> +		     (void *)base, size);

As we discussed via chat, doing it this way sucks the full content of
the log back into Open Firmware. 

That relies on OF handling such big properties, and also means more
memory will be consumed, which can cause problems early in boot.

A better solution is to explicitly add the log to the FDT in the
flattening phase.

Also adding the new linux,sml-log property should be accompanied by a
change to the device tree binding.

The syntax is not very obvious to me, but possibly something like?

diff --git a/Documentation/devicetree/bindings/tpm/ibm,vtpm.yaml b/Documentation/devicetree/bindings/tpm/ibm,vtpm.yaml
index 50a3fd31241c..cd75037948bc 100644
--- a/Documentation/devicetree/bindings/tpm/ibm,vtpm.yaml
+++ b/Documentation/devicetree/bindings/tpm/ibm,vtpm.yaml
@@ -74,8 +74,6 @@ required:
   - ibm,my-dma-window
   - ibm,my-drc-index
   - ibm,loc-code
-  - linux,sml-base
-  - linux,sml-size
 
 allOf:
   - $ref: tpm-common.yaml#
diff --git a/Documentation/devicetree/bindings/tpm/tpm-common.yaml b/Documentation/devicetree/bindings/tpm/tpm-common.yaml
index 3c1241b2a43f..616604707c95 100644
--- a/Documentation/devicetree/bindings/tpm/tpm-common.yaml
+++ b/Documentation/devicetree/bindings/tpm/tpm-common.yaml
@@ -25,6 +25,11 @@ properties:
       base address of reserved memory allocated for firmware event log
     $ref: /schemas/types.yaml#/definitions/uint64
 
+  linux,sml-log:
+    description:
+      Content of firmware event log
+    $ref: /schemas/types.yaml#/definitions/uint8-array
+
   linux,sml-size:
     description:
       size of reserved memory allocated for firmware event log
@@ -53,15 +58,22 @@ dependentRequired:
   linux,sml-base: ['linux,sml-size']
   linux,sml-size: ['linux,sml-base']
 
-# must only have either memory-region or linux,sml-base
+# must only have either memory-region or linux,sml-base/size or linux,sml-log
 # as well as either resets or reset-gpios
 dependentSchemas:
   memory-region:
     properties:
       linux,sml-base: false
+      linux,sml-log: false
   linux,sml-base:
     properties:
       memory-region: false
+      linux,sml-log: false
+  linux,sml-log:
+    properties:
+      memory-region: false
+      linux,sml-base: false
+      linux,sml-size: false
   resets:
     properties:
       reset-gpios: false

cheers