This is a draft change to discuss the idea of using the multipart
payload to implement a unified interface for LBR on Intel and AMD, and
BRBE on ARM.
Sponsored by: Netflix
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Hello folks this is a quick draft I threw together to sketch out providing a unified interface for branch histories like LBR on Intel and AMD, and BRBE on ARM. I looped in everyone that's working on a relevant patch.
The basic idea is that we introduce a new callback inside the add_sample path to allow for the branch payload to be filled in. This will use the multipart functions (so I need to update their scope). You can either fill in the payload into the callstack buffer and call pmc_multipart_add() or pmc_multipart_copydata() to copy a buffer into the stack frame. I did it this way because the branch history data is fairly large and we can't copy it on the stack.
From the user's perspective they just use the -B flag. E.g. pmcstat -B -S instructions -O samples.out.
(Please ignore that I didn't fill in all the getopt stuff again just a draft to facilitate discussion)