Messages in this thread | | | Date | Wed, 10 Jan 2018 10:22:34 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code |
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On Tue, Jan 09, 2018 at 06:02:53PM -0800, Dave Hansen wrote: > On 01/09/2018 05:06 PM, Thomas Gleixner wrote: > > --- a/arch/x86/kernel/cpu/bugs.c > > +++ b/arch/x86/kernel/cpu/bugs.c > > @@ -79,6 +79,7 @@ enum spectre_v2_mitigation_cmd { > > SPECTRE_V2_CMD_RETPOLINE, > > SPECTRE_V2_CMD_RETPOLINE_GENERIC, > > SPECTRE_V2_CMD_RETPOLINE_AMD, > > + SPECTRE_V2_CMD_IBRS, > > }; > > A few nits on this: > > IBRS should not default on anywhere, which goes double when retpolines > are available. > > I think I'd also prefer that we separate the IBRS and retpoline enabling > so that you can do both if you want. They do nearly the same thing in > practice, but I can't convince myself that you never ever need IBRS once > retpolines are in place.
As per:
https://lkml.kernel.org/r/1515460999.4423.104.camel@amazon.co.uk
IBRS=2 (always on) is preferred for SKL+ over retpoline.
And from what I gather IBRS=1 is never better than retpoline, IBRS=1 is both slower and covers less AFAIU (please educate if I'm wrong).
From this point, I would prefer to not even have the IBRS=1 code.
The only question I have is if retpoline works at all on SKL (with ucode update); BDW needs the ucode update for retpoline to work because of the RSB fallback.
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