Re: [PATCH v2 1/2] powerpc/tpm: Create linux,sml-base/size as big endian

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Wed Jul 12, 2023 at 12:39 PM UTC, Michael Ellerman wrote:
> Jarkko Sakkinen <jarkko@kernel.org> writes:
> > On Tue, 2023-07-11 at 08:47 -0400, Stefan Berger wrote:
> >> On 7/10/23 17:23, Jarkko Sakkinen wrote:
> >> > On Thu, 2023-06-15 at 22:37 +1000, Michael Ellerman wrote:
> >> > > There's code in prom_instantiate_sml() to do a "SML handover" (Stored
> >> > > Measurement Log) from OF to Linux, before Linux shuts down Open
> >> > > Firmware.
> >> > > 
> >> > > This involves creating a buffer to hold the SML, and creating two device
> >> > > tree properties to record its base address and size. The kernel then
> >> > > later reads those properties from the device tree to find the SML.
> >> > > 
> >> > > When the code was initially added in commit 4a727429abec ("PPC64: Add
> >> > > support for instantiating SML from Open Firmware") the powerpc kernel
> >> > > was always built big endian, so the properties were created big endian
> >> > > by default.
> >> > > 
> >> > > However since then little endian support was added to powerpc, and now
> >> > > the code lacks conversions to big endian when creating the properties.
> >> > > 
> >> > > This means on little endian kernels the device tree properties are
> >> > > little endian, which is contrary to the device tree spec, and in
> >> > > contrast to all other device tree properties.
> >> > > 
> >> > > To cope with that a workaround was added in tpm_read_log_of() to skip
> >> > > the endian conversion if the properties were created via the SML
> >> > > handover.
> >> > > 
> >> > > A better solution is to encode the properties as big endian as they
> >> > > should be, and remove the workaround.
> >> > > 
> >> > > Typically changing the encoding of a property like this would present
> >> > > problems for kexec. However the SML is not propagated across kexec, so
> >> > > changing the encoding of the properties is a non-issue.
> >> > > 
> >> > > Fixes: e46e22f12b19 ("tpm: enhance read_log_of() to support Physical TPM event log")
> >> > > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> >> > > Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
> >> > > ---
> >> > >   arch/powerpc/kernel/prom_init.c |  8 ++++++--
> >> > >   drivers/char/tpm/eventlog/of.c  | 23 ++++-------------------
> >> > >   2 files changed, 10 insertions(+), 21 deletions(-)
> >> > 
> >> > Split into two patches (producer and consumer).
> >> 
> >> I think this wouldn't be right since it would break the system when only one patch is applied since it would be reading the fields in the wrong endianess.
> >
> > I think it would help if the commit message would better explain
> > what is going on. It is somewhat difficult to decipher, if you
> > don't have deep knowledge of the powerpc architecture.
>
> I mean, it's already 8 paragraphs ¯\_(ツ)_/¯
>
> But I'm happy to expand it. I just don't really know what extra detail
> is needed to make it clearer.

Adding more text is not the right way to clarify things. I'd start
by explaining shortly SML and then move to the handover. It can't
be that hard, right?

Just adding new paragraphs would probably just make it even more
confusing.

BR, Jarkko