Re: [Xen-devel] [PATCH 0/3] x86: Full support of PAT

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Aug 26, 2014 at 1:08 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
> On Tue, 2014-08-26 at 11:48 -0700, Andy Lutomirski wrote:
>> On 08/25/2014 11:16 PM, Juergen Gross wrote:
>> > The x86 architecture offers via the PAT (Page Attribute Table) a way to
>> > specify different caching modes in page table entries. The PAT MSR contains
>> > 8 entries each specifying one of 6 possible cache modes. A pte references 
>> > one
>> > of those entries via 3 bits: _PAGE_PAT, _PAGE_PWT and _PAGE_PCD.
>> >
>> > The Linux kernel currently supports only 4 different cache modes. The PAT 
>> > MSR
>> > is set up in a way that the setting of _PAGE_PAT in a pte doesn't matter: 
>> > the
>> > top 4 entries in the PAT MSR are the same as the 4 lower entries.
>> >
>> > This results in the kernel not supporting e.g. write-through mode. 
>> > Especially
>> > this cache mode would speed up drivers of video cards which now have to use
>> > uncached accesses.
>> >
>> > OTOH some old processors (Pentium) don't support PAT correctly and the Xen
>> > hypervisor has been using a different PAT MSR configuration for some time 
>> > now
>> > and can't change that as this setting is part of the ABI.
>> >
>> > This patch set abstracts the cache mode from the pte and introduces tables 
>> > to
>> > translate between cache mode and pte bits (the default cache mode "write 
>> > back"
>> > is hard-wired to PAT entry 0). The tables are statically initialized with
>> > values being compatible to old processors and current usage. As soon as the
>> > PAT MSR is changed (or - in case of Xen - is read at boot time) the tables 
>> > are
>> > changed accordingly. Requests of mappings with special cache modes are 
>> > always
>> > possible now, in case they are not supported there will be a fallback to a
>> > compatible but slower mode.
>>
>> I feel like I'm missing something here.  Where's the support for the
>> high PAT bit on huge pages?  Once you start using the top four entries,
>> you'll need that.
>
> pgprot_4k_2_large() and pgprot_large_2_4k() provide the conversion of
> the PAT bit.

Whoops, I missed that.

>
>> Also, this probably needs errata handling.  IIRC there are a handful of
>> CPUs that support PAT but don't work correctly if the high bit is set.
>
> This patchset provides the infrastructure, but does not actually use the
> upper four entries.  I am working on additional patchset on top of this,
> which enables WT with the PAT bit except on the following Intel
> processors.  If I missed some processors affected, please let me know.
>
>   errata           cpuid
>   --------------------------------------
>   Pentium 2, A52   family 0x6, model 0x5
>   Pentium 3, E27   family 0x6, model 0x7
>   Pentium M, Y26   family 0x6, model 0x9
>   Pentium 4, N46   family 0xf, model 0x0
>

From very vague memory, that sounds correct.

--Andy

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