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SubjectRe: [PATCH v2 2/6] x86, mm, pat: Change reserve_memtype() to handle WT
On Wed, Sep 10, 2014 at 2:11 PM, Toshi Kani <toshi.kani@hp.com> wrote:
> On Wed, 2014-09-10 at 14:06 -0700, Andy Lutomirski wrote:
>> On Wed, Sep 10, 2014 at 1:30 PM, Toshi Kani <toshi.kani@hp.com> wrote:
>> > On Wed, 2014-09-10 at 13:14 -0700, H. Peter Anvin wrote:
>> >> On 09/10/2014 12:30 PM, Toshi Kani wrote:
>> >> >
>> >> > When WT is unavailable due to the PAT errata, it does not fail but gets
>> >> > redirected to UC-. Similarly, when PAT is disabled, WT gets redirected
>> >> > to UC- as well.
>> >> >
>> >>
>> >> But on pre-PAT hardware you can still do WT.
>> >
>> > Yes, if we manipulates the bits directly, but such code is no longer
>> > allowed for PAT systems. The PAT-based kernel interfaces won't work for
>> > pre-PAT systems, and therefore requests are redirected to UC- on such
>> > systems.
>> >
>>
>> Right, the PWT bit. Forgot about that.
>>
>> I wonder whether it would make sense to do some followup patches to
>> replace the current support for non-PAT machines with a "PAT" and
>> corresponding reverse map that exactly matches the mapping when PAT is
>> disabled. These patches are almost there.
>
> That's possible, but the only benefit is that we can enable WT on
> pre-PAT systems, which I do not think anyone cares now... WB & UC work
> on pre-PAT systems. WC & WT need PAT. I think this requirement is
> reasonable.

It might end up being a cleanup, though. A whole bunch of
rarely-exercised if (!pat_enabled) things would go away.

--Andy


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