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

[Alan Cox]

On Fri, 26 Jan 2018 13:14:46 +0100
Yves-Alexis Perez <corsac@xxxxxxxxxx> 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 âdefault onâ? 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