RE: [PATCH] 2.6 workaround for Athlon/Opteron prefetch errata

David Lang
2003-09-16T00:04:51Z

On Mon, 15 Sep 2003 richard.brunner@amd.com wrote:

> My concern is trying to prevent
> the flood of emails where someone thinks they built
> a "standard" kernel only to discover that they forgot
> to select the various suboptions
> and it doesn't work on their processor. I'd like
> to simplfy what the majority of folks need to do
> to get a broadly working kernel.

I agree, and if the kernel reports a CPU/build mismatch as early in the
boot process as possible this would seem to solve this problem.

we already get a steady stream of such messages and so being able to add
this check would cut down on the support load now.

the problem seems to be that currently we don't get the ability to display
anything until fairly late in the boot process, so reporting this cleanly
may require adding another chunk of code (thankfully code that can be
thrown away as soon as it's used) to do this checking.

or another option would be to make the 'configured CPU' mask something
that bootloaders can check before they load the kernel (but that would
mean that the bootloader would have to detect the CPU type)

David Lang

> I'd prefer that some set of options "take away"
> rather than "add" features/work-arounds. In the case below,
> I'd prefer a "don't support Athlon Prefetch Errata".
>
> I think you can argue that some features are going to
> be common enough for the majority of users that they
> should be in the "take-away" category.
>
> Others are uncommon enough that they fall into
> the "add" category.
>
> If you stick with consistent nomenclature
> (e.g. add_feature_TBD & sub_default_feature_TBD)
> and put these options in one common config file,
> I think the embedded folks and the
> "optimize every last bit" folks get
> what they want.
>
> ] -Rich ...
> ] AMD Fellow
> ] richard.brunner at amd com
>
> > -----Original Message-----
> > From: Bill Davidsen [mailto:davidsen@tmr.com]
> > Sent: Monday, September 15, 2003 12:16 PM
> > To: Brunner, Richard
> > Cc: alan@lxorguk.ukuu.org.uk; zwane@linuxpower.ca;
> > linux-kernel@vger.kernel.org
> > Subject: RE: [PATCH] 2.6 workaround for Athlon/Opteron prefetch errata
> >
> >
> > On Mon, 15 Sep 2003 richard.brunner@amd.com wrote:
> >
> > > I think Alan brought up a very good point. Even if you
> > > use a generic kernel that avoids prefetch use on Athlon
> > > (which I am opposed to), it doesn't solve the problem
> > > of user space programs detecting that the ISA supports
> > > prefetch and using prefetch instructions and hitting the
> > > errata on Athlon.
> > >
> > > The user space problem worries me more, because the expectation
> > > is that if CPUID says the program can use perfetch, it could
> > > and should regardless of what the kernel decided to do here.
> > >
> > > Andi's patch solves both the kernel space and the user space
> > > issues in a pretty small footprint.
> >
> > Clearly AMD would like to avoid having PIV and Athlon
> > optimized kernels,
> > and to default to adding unnecessary size to the PIV kernel to support
> > errata in the Athlon. But fighting against having a config
> > which produces
> > a smaller and faster kernel for all non-Athlon users and all
> > embedded or
> > otherwise size limited users seems to be just a marketing
> > thing so P4 code
> > will seem to work correctly on Athlon.
> >
> > Vendors will build a kernel which runs as well as possible on
> > as many CPUs
> > as possible, but users who build their own kernel want to
> > build a kernel
> > for a particular config in most cases and should have the
> > option. There
> > should be a "support Athlon prefetch" option as well, which turns on
> > the fix only when it's needed, just as there is for P4
> > thermal throttling,
> > F.P. emulation, etc. Why shouldn't this be treated the same
> > way as other
> > features already in the config menu?
> >
> > --
> > bill davidsen <davidsen@tmr.com>
> > CTO, TMR Associates, Inc
> > Doing interesting things with little computers since 1979.
> >
> >
>
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