Re: [PATCH] i386, x86_64 Initial PAT implementation

[ebiederm@xmission.com (Eric W. Biederman)]

Mikael Pettersson <mikpe@csd.uu.se> writes:

> 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.)

It is still useful even if that is the case.  Unless we need something
more than WC we don't have a need for anything else.  

If you could track down the reference you are thinking of I would
appreciate it.  According to the errata I can find simply not using
the high 4 entries is sufficient, to avoid the problem.

Eric


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