On Thu, 31 Jan 2002, Kenneth Culver wrote:
> Yeah, that's what I saw on linux-kernel...
You probably didn't see the whole story or just did a too selective
reading. You should re-read, in my opinion, since you may have just missed
the important part.
What I understood is that the Athlon allocates cache lines for speculative
writes (if write may hit cachable) and such cache lines are flushed to
memory even if the write does not happen. As a result, numerous useless
writes to memory may happen under any OS.
This does not cause visible issue for memory that is required to be cache
coherent. Just AGP accesses are not required to be snooped by cache for
performance concerns and as memory given to AGP is intended to contain
data as textures that should not need tight synchronisation with CPU.
Indeed Linux has cached mapping to AGP memory and this could be avoided.
Nevertheless, only strange issues like the AMD AGP bug we are talking
about could turn this into a real problem.
Linux can be fixed, but the useless writes of the existing Athlons from
the very fast cache to the relatively very slow memory cannot. And all
Athlon users may well pay this penalty under any OS... unless we want to
disable caching. :)
Btw, I have 2 Athlon 1.2GHz machines that work just fine and fast for me.
> On Thu, 31 Jan 2002, Cameron, Frank wrote:
> > From what was posted on the linux-kernel list the problem is the OS
> > doing the wrong thing not the hardware. I originally asked the
> > question (albeit not worded as clearly as I should have) because if
> > Microsoft and Linux programmers made the same mistake, might
> > FreeBSD have also.
> > > -----Original Message-----
> > > From: Kenneth Culver [mailto:[EMAIL PROTECTED]]
> > > Sent: Thursday, January 31, 2002 10:42 AM
> > > To: Terry Lambert
> > > Cc: David Malone; Cameron, Frank; '[EMAIL PROTECTED]';
> > > '[EMAIL PROTECTED]'
> > > Subject: Re: AMD AGP Bug
> > >
> > >
> > > > There's actually a seperate TLB bug, but FreeBSD doesn't
> > > > trigger that one, either (Linux can tickle it, when there
> > > > are certain specific circumstances met).
> > > >
> > > Well, I think I know what you're talking about, linux
> > > allocates agpgart
> > > memory without setting a "non-cacheable" bit, and then the
> > > agp card writes
> > > to that memory, but the cpu cached it already, which makes
> > > the cache wrong
> > > or something like that, and causes the crashes/hangs. I know this is a
> > > greatly simplified version of the real problem, but I think this is a
> > > linux bug not necesarily an amd bug.
> > >
> > > Ken
> > >
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