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Date: Mon, 8 Jan 2018 20:28:53 -0200
From: Eduardo Habkost <ehabkost@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] target-i386: add pcid to both Sandy Bridge
 and Ivy Bridge
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To: Vincent Bernat <vincent@bernat.im>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>, qemu-devel@nongnu.org

On Mon, Jan 08, 2018 at 11:22:36PM +0100, Vincent Bernat wrote:
>  =E2=9D=A6  8 janvier 2018 20:14 -0200, Eduardo Habkost <ehabkost@redha=
t.com>=C2=A0:
>=20
> >> What are the consequences of running a too old kernel? Would KVM jus=
t
> >> hide PCID flag?
> >
> > On an old kernel, the SandyBridge and IvyBridge CPU models will
> > be unexpectedly become not runnable.
>=20
> But, isn't it the same for more recent models that already have PCID
> enabled?

Yes, the more recent models are already not runnable on those
hosts.  The key here is "unexpectedly": management software can
assume that the CPU model won't become unrunnable when it was
runnable in the past, and logic that decides if/where a VM can be
started (or migrated to) might break.

--=20
Eduardo