On Tue, Oct 21, 2025 at 01:20:50PM -0400, Adam Young wrote:

[...]

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Because the PCC spec is wonky.
https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/14_Platform_Communications_Channel/Platform_Comm_Channel.html#extended-pcc-subspace-shared-memory-region

"Length of payload being transmitted including command field."  Thus in
order to copy all of the data, including  the PCC header, I need to drop the
length (- sizeof(u32) ) and then add the entire header. Having all the PCC
data in the buffer allows us to see it in networking tools. It is also
parallel with how the messages are sent, where the PCC header is written by
the driver and then the whole message is mem-copies in one io/read or write.

No you have misread this part.
Communication subspace(only part and last entry in shared memory at offset of
16 bytes) - "Memory region for reading/writing PCC data. The maximum size of
this region is 16 bytes smaller than the size of the shared memory region
(specified in the Master slave Communications Subspace structure). When a
command is sent to or received from the platform, the size of the data in
this space will be Length (expressed above) minus the 4 bytes taken up by
the command."

The keyword is "this space/region" which refers to only the communication
subspace which is at offset 16 bytes in the shmem.

It should be just length - sizeof(command) i.e. length - 4

I just want to make sure I have this correct.  I want to copy the entire PCC
buffer, not just the payload, into the sk_buff.  If I wanted the payload, I
would use the length field.  However, I want the PCC header as well, which
is the length field, plus sizeof (header).  But that double counts the
command field, which is part of the header, and thus I subtract this out.  I
think my math is correct. What you wrote would be for the case where I want
only the PCC payload.

Why ? How does sk_buff interpret that as PCC header. Something doesn't align
well here or I might be missing something.

I started writing some helpers and this comment made me to rethink my
approach. I don't have any to share and I will be away for a while. I will
try to review any further changes from you but expect delays.

-- 
Regards,
Sudeep