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

[Toshi Kani <toshi.kani@xxxxxx>]

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.

> 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

Thanks,
-Toshi


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