Disabling Spectre V2 Mitigations Is What Can Impair AMD Ryzen 7000 Series Performance

[sabian2008]

Isn't it possible that there's a kernel bug or that it's less optimized? It isn't impossible but it sounds weird that removing mitigations hurts performance. And we've seen in the past that somewhat new processors had lots of weird scheduling and clocking regressions. Is the same effect observed in Windows or *BSD?

[birdie]

There's some trickery going on, e.g. the CPU could completely discard the instructions which are meant to protect against Spectre but there are other protections in place so the vulnerability is taken care of. Amazing I'd say except this could open the door to new vulnerabilities.

So now instead of being able to disable mitigations and get the processor's full-performance, the mitigations are built-in, presumably causing the same impact, and now can't be disabled?

[EvilHowl]

My theory is that fixing the Spectre V2 vulnerability on a hardware level would lead to fundamental architecture changes that AMD is not willing to make, because it may add so much more complexity to the architecture or it may just be too unconvenient. They probably realized that optimizing the code paths that the Linux kernel utilizes on the default mitigations mode is faster, simpler and it may involve less deeper changes, while still being secure.

As far as I know, pretty much every CPU architecture that implements speculative execution is vulnerable to some version of Spectre, so note that this is not a fundamental flaw of AMD64.

[phoronix]

Phoronix: Disabling Spectre V2 Mitigations Is What Can Impair AMD Ryzen 7000 Series Performance

Last week I shared some initial numbers how surprisingly when disabling Zen 4 CPU security mitigations can actually *hurt* the Ryzen 7000 series CPU performance. While conventional wisdom and with past Intel/AMD processors yield better performance when disabling the CPU security mitigations, with the Ryzen 9 7950X it was found to be basically the opposite. I have since conducted more tests and using an AMD Ryzen 5 7600X to confirm the earlier results and dig deeper into the data.

https://www.phoronix.com/review/amd-zen4-spectrev2