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

From: Peter Zijlstra
Date: Thu Jan 25 2018 - 04:43:16 EST


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;
> +}