Re: [PATCH] i386, x86_64 Initial PAT implementation

[Mikael Pettersson]

Andi Kleen writes:
 > On Tuesday 30 August 2005 16:45, Alan Cox wrote:
 > > On Llu, 2005-08-29 at 18:20 -0600, Eric W. Biederman wrote:
 > > > ways.  Currently this code only allows for an additional flavor
 > > > of uncached access to physical memory addresses which should be hard
 > > > to abuse, and should raise no additional aliasing problems.  No
 > > > attempt has been made to fix theoretical aliasing problems.
 > >
 > > Even an uncached/cached alias causes random memory corruption or an MCE
 > > on x86 systems. In fact it can occur even for an alias not in theory
 > > touched by the CPU if it happens to prefetch into or speculate the
 > > address.
 > >
 > > Also be sure to read the PII Xeon errata - early PAT has a bug or two.
 > 
 > 
 > We can always force cpu_has_pat == 0 on these machines.
 > I don't think it is worth it to add any more complicated workarounds 
 > for old broken systems.

I don't have the spec updates in front of me, but I believe the PAT
bug existed well into the P4 line. The workaround is simply to make
the high 4 PAT entries identical to the low 4 entries. (But I confess
to not having a clue as to whether it's still useful then or not.)
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