Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure

[Eduardo Habkost <ehabkost@redhat.com>]

On Sat, Jan 20, 2018 at 08:22:56PM +0100, KarimAllah Ahmed wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Not functional yet; just add the handling for it in the Spectre v2
> mitigation selection, and the X86_FEATURE_IBRS flag which will control
> the code to be added in later patches.
> 
> Also take the #ifdef CONFIG_RETPOLINE from around the RSB-stuffing; IBRS
> mode will want that too.
> 
> For now we are auto-selecting IBRS on Skylake. We will probably end up
> changing that but for now let's default to the safest option.
> 
> XX: Do we want a microcode blacklist?
> 
> [karahmed: simplify the switch block and get rid of all the magic]
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
[...]
> +	case SPECTRE_V2_CMD_FORCE:
> +		/*
> +		 * If we have IBRS support, and either Skylake or !RETPOLINE,
> +		 * then that's what we do.
> +		 */
> +		if (boot_cpu_has(X86_FEATURE_SPEC_CTRL) &&
> +		    (is_skylake_era() || !retp_compiler())) {


Sorry for being confused here, as probably the answer is buried
on a LKML thread somewhere.  The comment explains what the code
does, but not why.  Why exactly IBRS is preferred on Skylake?

I'm asking this because I would like to understand the risks
involved when running under a hypervisor exposing CPUID data that
don't match the host CPU.  e.g.: what happens if a VM is migrated
from a Broadwell host to a Skylake host?



> +			mode = SPECTRE_V2_IBRS;
> +			setup_force_cpu_cap(X86_FEATURE_IBRS);
> +			break;
> +		}
> +		/* Fall through */
>  	case SPECTRE_V2_CMD_RETPOLINE:
[...]

-- 
Eduardo