Only kinda effective. Retpoline on Skylake and later isn't completely reliable as a mitigation for one of the spectre variants, and IBRS is one of the processor

Only kinda effective. Retpoline on Skylake and later isn't completely reliable as a mitigation for one of the spectre variants, and IBRS is one of the processor features that will supposedly correct that. You'll still be vulnerable, it'll just add another layer of difficulty to the exploit.