Re: [PATCH v1 3/3] x86/bugs: Switch the selection of mitigation from CPU vendor to CPU features

[Tom Lendacky]

On 6/1/2018 9:59 AM, Konrad Rzeszutek Wilk wrote:
> Both AMD and Intel can have SPEC CTRL MSR for SSBD.
> 
> However AMD also has two more other ways of doing it - which
> are !SPEC_CTRL MSR ways.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> 
> ---
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/bugs.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 6bea81855cdd..cd0fda1fff6d 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -532,17 +532,12 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
>  		 * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD may
>  		 * use a completely different MSR and bit dependent on family.
>  		 */
> -		switch (boot_cpu_data.x86_vendor) {
> -		case X86_VENDOR_INTEL:
> -		case X86_VENDOR_AMD:
> -			if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
> -				x86_amd_ssb_disable();
> -				break;
> -			}
> +		if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
> +			x86_amd_ssb_disable();
> +		else {

As I think about this more, I don't think we can do this for AMD.  The
X86_FEATURE_SSBD could be true because of the LS_CFG support and not the
AMD_SSBD CPUID bit.  But if the IBRS CPUID bit was set, we would now try
to use the SPEC_CTRL register for SSBD, which is not valid.

I think you will have to keep the case statements and explicitly check for
X86_FEATURE_AMD_SSBD before using SPEC_CTRL.

Thanks,
Tom

>  			x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
>  			x86_spec_ctrl_mask |= SPEC_CTRL_SSBD;
>  			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> -			break;
>  		}
>  	}
>  
>