Messages in this thread | | | Date | Wed, 20 Oct 2021 17:18:08 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE* |
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On Wed, Oct 20, 2021 at 05:07:53PM -0700, Alexei Starovoitov wrote: > On Wed, Oct 20, 2021 at 12:44:56PM +0200, Peter Zijlstra wrote: > > + > > + if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) { > > + EMIT_LFENCE(); > > + EMIT2(0xFF, 0xE0 + reg); > > + } else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) { > > + emit_jump(&prog, reg_thunk[reg], ip); > > + } else > > One more question. > What's a deal with AMD? I thought the retpoline is effective on it as well. > lfence is an optimization or retpoline turned out to be not enough > in some cases?
Yes, it's basically an optimization. AMD recommends it presumably because it's quite a bit faster than a retpoline.
According to AMD it shrinks the speculative execution window enough so that Spectre v2 isn't a threat.
-- Josh
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