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

On Fri, Mar 08, 2024 at 07:23:35AM -0500, Stefan Berger wrote:

On 3/7/24 16:52, Rob Herring wrote:

quoted

On Thu, Mar 07, 2024 at 09:41:31PM +1100, Michael Ellerman wrote:

quoted

Stefan Berger [off-list ref] writes:

quoted

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 <stefanb@linux.ibm.com>
---
  arch/powerpc/kernel/prom_init.c | 8 ++------
  1 file changed, 2 insertions(+), 6 deletions(-)

quoted

quoted

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

Dropping required properties is an ABI break. If you drop them, an older
OS version won't work.

1) On PowerVM and KVM on Power these two properties were added in the Linux
code. I replaced the creation of these properties with creation of
linux,sml-log (1/2 in this series). I also replaced the handling of
these two (2/2 in this series) for these two platforms but leaving it for
powernv systems where the firmware creates these.

Okay, I guess your case is not a ABI break if the kernel is populating 
it and the same kernel consumes it. 

You failed to answer my question on using /reserved-memory. Again, why 
can't that be used? That is the standard way we prevent chunks of memory 
from being clobbered. There's already support for describing the TPM log 
that way anyways. The only reasoning I can see writing out a node for 
that is harder than just adding a property, but that's not a great 
argument IMO.

2) There is an example in the ibm,vtpm.yaml file that has both of these
and the test case still passes the check when the two entries above are
removed. I will post v2 with the changes to the DT bindings for
linux,sml-log including an example for linux,sml-log. [The test cases fail,
as expected, when an additional property is added, such as when
linux,sml-base is added when linux,sml-log is there or linux,sml-log is
added when linux,sml-base is there.]

Sure, removing a required property is never going to break the DT 
checks. What would break is a client (OS) version that only understands 
linux,sml-base and can no longer get the log assuming getting the log 
itself was required. 

Rob