[loeg]

AMD still takes the hit for Spectre mitigations, even if it does not need Meltdown mitigation.

And I'm not sure it's possible to fairly discount Intel's performance with meltdown mitigations applied. I think the impact will vary depending on workload.

[dmichulke]

> AMD still takes the hit for Spectre mitigations, even if it does not need Meltdown mitigation.

That's why I wanted to keep it out of the discussion and just mentioned Meltdown, AFAIK Spectre applies to both so it would be pure speculation to identify who'll be hit harder.

> And I'm not sure it's possible to fairly discount Intel's performance with meltdown mitigations applied. I think the impact will vary depending on workload.

I think we have this problem already all the time (with or without mitigations applied), that's why we (should) interpret benchmarks only as a proxy.

[loeg]

> That's why I wanted to keep it out of the discussion and just mentioned Meltdown, AFAIK Spectre applies to both so it would be pure speculation to identify who'll be hit harder.

That makes sense. Many people conflate the two, so I just wanted to be explicit about what I was saying :-).

> I think we have this problem already all the time (with or without mitigations applied), that's why we (should) interpret benchmarks only as a proxy.

That's totally reasonable. I think there were some discussions of the impact of the Meltdown patch (on Intel performance) on the LKML list at the time the patch(es) were being reviewed. (Other OS may have different perf impact for their Meltdown mitigations, of course, but it helps ballpark.)

Here's some discussion on anandtech, although it doesn't measure Spectre mitigations alone vs Spectre+MD; only base, MD alone, and MD+Spectre:

https://www.anandtech.com/show/12566/analyzing-meltdown-spec...

[spacenick88]

Afaik not, at least on Linux the mitigations are turned on as needed by CPU ID data