Re: RFC: CXL: How to handle trace event ABI break from CXL_HEADERLOG_SIZE fix?

["Dan Williams (nvidia)" <djbw@kernel.org>]

Bowman, Terry wrote:
> Hi Mauro and Everyone,
> 
> During development of the CXL protocol series [1], the Sashiko tool 
> identified an issue in the CXL RAS UCE trace logging for Ports and Endpoints. 
> Specifically, the CXL RAS UCE header log size is incorrectly defined in 
> drivers/cxl/cxl.h.
> 
> The UCE header log size is currently defined as 128 u32s (512 bytes),
> whereas it should be 16 u32s (64 bytes) per CXL r4.0 8.2.4.17.7. Correcting 
> this will change the trace format and break the existing ABI contract 
> with rasdaemon.
> 
> How would you recommend proceeding to resolve this?

Split the definitions into one that reads from the register space and
once that passes the parsed log to the trace buffer.

Something like:

u32 hl[CXL_HEADERLOG_TRACE_SIZE_U32];

...zero-fill that and put a comment about how userspace has already
grown a dependency on the buggy size.

In other words, keep the userspace compatibility, but fix the iomem
overflow.