Re: [PATCH v2 5/5] x86/pti: Do not enable PTI on fixed Intel processors

Andi Kleen
2018-01-23T18:12:01Z

>
> - if (c->x86_vendor != X86_VENDOR_AMD)
> - setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
> + if (c->x86_vendor != X86_VENDOR_AMD) {
> + u64 ia32_cap = 0;
> +
> + if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
> + rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
> + if (!(ia32_cap & ARCH_CAP_RDCL_NO))
> + setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);

This means that in a hypervisor which passes through the CPUID,
but actually doesn't implement the MSR (so rdmsr #GPs and returns 0)
it would be cleared.

It would be better to usr rdmsrl_safe and check the return value.

-Andi