The INVLPG instruction is used to invalidate TLB entries for a
specified virtual address.  When PCIDs are enabled, INVLPG is supposed
to invalidate TLB entries for the specified address for both the
current PCID *and* Global entries.  (Note: Only kernel mappings set
Global=1.)

Unfortunately, some INVLPG implementations can leave Global
translations unflushed when PCIDs are enabled.

As a workaround, never enable PCIDs on affected processors.

I expect there to eventually be microcode mitigations to replace this
software workaround.  However, the exact version numbers where that
will happen are not known today.  Once the version numbers are set in
stone, the processor list can be tweaked to only disable PCIDs on
affected processors with affected microcode.

Note: if anyone wants a quick fix that doesn't require patching, just
stick 'nopcid' on your kernel command-line.
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org