linux-kernel - Re: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown

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Date:   Fri, 26 Jan 2018 13:14:46 +0100
From:   Yves-Alexis Perez <corsac@...ian.org>
To:     David Woodhouse <dwmw@...zon.co.uk>, 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, dave.hansen@...el.com,
        gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which
 are not vulnerable to Meltdown

On Wed, 2018-01-24 at 16:57 +0000, David Woodhouse wrote:
> Some old Atoms, anything in family 5 or 4, and newer CPUs when they advertise
> the IA32_ARCH_CAPABILITIES MSR and it has the RDCL_NO bit set, are not vulnerable.
> 
> Roll the AMD exemption into the x86_match_cpu() table too.
> 
> Based on suggestions from Dave Hansen and Alan Cox.

Hi David,

I know we'll still be able to manually enable PTI with a command line option,
but it's also a hardening feature which has the nice side effect of emulating
SMEP on CPU which don't support it (e.g the Atom boxes above).

Couldn't we keep the “default on”? Or maybe on boxes which also have CPID (in
order to limit the performance cost)?

Regards,
-- 
Yves-Alexis
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