Intrepid compatibility with C3 CPUs #5212

[Colin Watson]

On Thu, Aug 21, 2008 at 01:49:27AM -0400, M G wrote:
> Subject: Intrepid compatibility with C3 CPUs #5212

Are you referring to a particular bug here?
https://launchpad.net/bugs/5212 is something quite different.

> The C3 is still listed as a current product on the Via web site.
> However, if Wikipedia can be trusted, then not all C3 processors are
> the same when it comes to cmov support. The CPU flags have to be
> examined to see if a particular generation of C3 model supports cmov
> or not. The problem also may not be simply the C3. There are other low
> power CPUs that may also be affected (e.g. Geode).

Is this a problem you're encountering personally? On what release of
Ubuntu are you encountering it?

The reason I ask is that I thought we already took some care to avoid
cmov, precisely because of this problem. In fact, before Ubuntu went
public, we had to purge the archive and rebuild it from scratch because
we realised that the use of cmov meant we wouldn't work on many C3 CPUs.

> To look at the problem in general though, there are several issues
> which must be considered.
> 
> 1) The Intrepid 32 bit kernel describes itself as i386
> (intrepid-desktop-i386.iso). This is clearly incorrect and misleading.

i386 is the architecture identifier, and cannot be changed (well, not
without an absurd amount of effort and creating other problems). I don't
propose to make the CD image file name not match the architecture name
because that would also be confusing to a different set of people.

Furthermore, "i386" is only visible in the file name. The *description*
reads as follows:

PC (Intel x86) desktop CD

    For almost all PCs. This includes most machines with Intel/AMD/etc
    type processors and almost all computers that run Microsoft Windows,
    as well as newer Apple Macintosh systems based on Intel processors.
    Choose this if you are at all unsure.

> 2) The download page states "Choose this if you are at all unsure".
> This is also clearly incorrect. Someone attempting to use this version
> must be absolutely sure what CPU they have, and possibly even the
> exact version.

If you can't run the i386 CD (and you aren't using something other than
a PC, like a pre-Intel Apple Mac or a SPARC; those people generally know
what they've got), then no other Ubuntu image is going to work for you.
As such I think the clear "this is what to do if the rest of the page is
just so much jargon to you" is entirely appropriate.

> 3) The error message from the installer is unhelpful. It asks the user
> to "please use a kernel appropriate for your CPU", yet there is no
> hint as to where this kernel may be found. What is the user supposed
> to do at this point?

If the user gets that message, there is nothing they can do except
report a bug (like many other installation problems).

Speaking of which, are you encountering this message? Please file a bug
report here:

  https://bugs.launchpad.net/ubuntu/+source/base-installer/+filebug

I'll need you to attach the contents of /proc/cpuinfo to your bug
report. (The rest of your message implies that you currently have Ubuntu
running on this machine, so you should be able to use that to get at
/proc/cpuinfo.)

> 4) Making this change will cause PCs which currently use Ubuntu to no
> longer be able to do so.
> 
> 5) Making this change will limit the hardware compatibility in ways
> which very few users (even very technically inclined ones) would
> expect.

I believe the change you're referring to here is that the Intrepid i386
-generic kernel started being compiled with CONFIG_M686 rather than the
previous CONFIG_M586. When I heard about this change, I raised it with
the kernel team manager (since we happened to be in the same room at the
time), who IIRC said that this was a mistake and was not intentional. It
appears to be fixed in the Intrepid 2.6.27 kernels.

However, this won't help the installer problem you seem to be having;
for that, I need a bug report with that information I mentioned above.

> The latter two items are the serious problems. Dropping users and
> dropping support for basic hardware isn't a decision to be made
> lightly, and it certainly shouldn't be made quietly by a few people
> and sprung on everyone else by surprise as a fait acompli. It's
> something that should be discussed more widely with respect to what
> markets are being served and what the overall support strategy should
> be.

If I'm understanding your message correctly, I think you're elevating a
mistake to the status of a major decision. :-)

> There are basically two types of x86 Ubuntu offered at present - 32
> bit and 64 bit. Most new CPUs sold today are 64 bit, so that is where
> the future of Ubuntu ought to be. 32 bit is for older (legacy)
> hardware or for specialised uses such as low power or embedded
> applications. In other words, 64 bit is for mainstream desktops,
> laptops and servers and 32 bit is for legacy and special purpose.

Without wishing to disagree with your conclusion, I think this premise
is mistaken. Due to the prevalence of proprietary code that has trouble
running on 64-bit systems (notably Flash, until nspluginwrapper came
along relatively recently, and there are others), many people still
prefer to use a 32-bit installation even if their system is able to run
64-bit code.

Cheers,

-- 
Colin Watson                                       [cjwatson at ubuntu.com]