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 16:47:47 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Yves-Alexis Perez <corsac@...ian.org>
Cc:     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
Subject: Re: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which
 are not vulnerable to Meltdown

On Fri, 26 Jan 2018 13:14:46 +0100
Yves-Alexis Perez <corsac@...ian.org> wrote:

> 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)?

For the old atom processors you really don't want the extra cost as a
default. These are older much slower devices and don't have PCID.

Alan