Re: [PATCH 2/2] x86: Add generic CPUID data dumper to ld.so --list-diagnostics

[Florian Weimer <fweimer@redhat.com>]

* Noah Goldstein:

> On Fri, Sep 8, 2023 at 3:10 PM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> This is surprisingly difficult to implement if the goal is to produce
>> reasonably sized output.  With the current approaches to output
>> compression (suppressing zeros and repeated results between CPUs,
>> folding ranges of identical subleaves, dealing with the %ecx
>> reflection issue), the output is less than 600 KiB even for systems
>> with 256 threads.
>>
> Maybe should just output a complete json?

JSON cannot directly represent 64-bit integers, so it would need some
custom transformation for other parts of the --list-diagnostics output.

> Then users can pretty easily write scripts to extract the exact information
> they are after. Or or the dumper can be extended in the future to let
> the user specify fields/values to dump so it can be configured to be more
> reasonable?

I'm not sure what is unreasonable about the current implementation?  I
complained about how hard it is getting the data and distilling it into
something that is not a gigantic data blob.

To be clear, without only trivial zero-values suppression, brute-force
enumeration (cutting off at 512 subleaves) results in roughly 8 KiB of
raw data per *CPU*.  It's even larger for recent CPUs which have more of
the funny ECX behavior (where unsupported subleaves do not come back as
zero).

Thanks,
Florian