AMD's Zen 4 has been tested with the various Spectre variant mitigations and the performance is surprisingly good. In fact, overall, the Ryzen 7950X has managed to beat the unpatched system.
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Intel and ARM are vulnerable to the Spectre-BHB flaw, but AMD is apparently troubled by Spectre v2, which it should have fixed back in 2018. AMD has now issued a new fix for the CVE-2017-5715 bug.