Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

[Dave Hansen]

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.