Re: [PATCH 2/5] x86: PAT: fix ambiguous paranoia check in pat_init()

On Wed, Jun 11, 2008 at 02:58:49PM +0200, Rene Herman wrote:
On 11-06-08 11:41, Andreas Herrmann wrote:

As I had no Transmeta or Centaur CPU at hand I just cleared the PAT
flag in the CPU identification code to simulate the case that all CPUs
of a Vendor are whitelisted (even those w/o PAT support). The first
time pat_init() is entered you get
PAT enabled, but CPU feature cleared (=> which is wrong as no flag
was cleared)

Again, you are misreading this. Please just replace the message mentally by
"PAT enabled, but CPU claims to not support PAT". "cleared" here does not
signify that we ourselves cleared anything, just that flag IS clear
(unset). Yes, maybe the wording could be better but it's not wrong.

Well, wording might not be best. But I don't care anymore.
(Just wondering which CPUs are out there that support PAT but don't
advertise it with any feature flag.)

x86 PAT enabled: cpu 0, old 0x7040600070406, new 0x7010600070106
(=> which is wrong as PAT shouldn't be enabled on such CPUs)

Again not wrong, or at least by design. Thomas Gleixner did it this way and
with that "paranoia check" explicitly bombing out only for SMP this
wouldn't have been by accident. He no doubt knows why he did so (and he's
in CC so if we're real lucky we might also now...)

I guess at the time Thomas' patch was commited this was just fine.

But with the recent Transmeta/Centaur patch, validate_pat_support()
returns w/o disabling PAT even for such vendor's CPUs that don't
support PAT,

To prevent this, validate_pat_support() should check for cpu_has_pat
in addition to any other white-black-or-whatsoever-listing.


Regards,

Andreas


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