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

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Date:   Wed, 24 Jan 2018 01:27:12 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>, arjan@...ux.intel.com,
        tglx@...utronix.de, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        bp@...en8.de, peterz@...radead.org, pbonzini@...hat.com,
        ak@...ux.intel.com, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, thomas.lendacky@....com
Subject: Re: [PATCH v2 5/5] x86/pti: Do not enable PTI on fixed Intel
 processors

On Tue, 2018-01-23 at 10:40 -0800, Dave Hansen wrote:
> 
> I'd really rather we break this out into a nice, linear set of
> true/false conditions.
> 
> bool early_cpu_vulnerable_meltdown(struct cpuinfo_x86 *c)
> {
>         u64 ia32_cap = 0;
> 
>         /* AMD processors are not subject to Meltdown exploit: */
>         if (c->x86_vendor == X86_VENDOR_AMD)
>                 return false;
> 
>         /* Assume all remaining CPUs not enumerating are vulnerable: */
>         if (!cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
>                 return true;
> 
>         /*
>          * Does the CPU explicitly enumerate that it is not vulnerable
>          * to Rogue Data Cache Load (aka Meltdown)?
>          */
>         rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
>         if (ia32_cap & ARCH_CAP_RDCL_NO)
>                 return false;
> 
>         /* Assume everything else is vulnerable */
>         return true;
> }

Makes sense. It also starts to address Alan's "starting to get messy"
comment, and gives a simple way to add other conditions.
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