Re: [PATCH v4 5/7] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 25, 2018 at 09:23:07AM +0000, David Woodhouse wrote:
> +static bool __init early_cpu_vulnerable_meltdown(struct cpuinfo_x86 *c)
> +{
> +	u64 ia32_cap = 0;
> +
> +	if (x86_match_cpu(cpu_no_meltdown))
> +                return false;
> +
> +	if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
> +		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);

I think it was suggested a while back to write this like:

	if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES) &&
	    !rdmsrl_safe(MSR_IA32_ARCH_CAPABILITIES, ia32_cap))

to deal with funny virt scenarios where they accidentally advertise the
CPUID bit but don't in fact provide the MSR.

> +
> +	/* Rogue Data Cache Load? No! */
> +	if (ia32_cap & ARCH_CAP_RDCL_NO)
> +		return false;
> +
> +	return true;
> +}