2.6 changesets

2.6 changesets

[James Harper]

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I was just browsing the latest changesets and noticed some references to 2.6. Is it anywhere near testable yet?

James 

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Re: 2.6 changesets

[Ian Pratt]

> I was just browsing the latest changesets and noticed some references to 2.6. Is it anywhere near testable yet?

It's currently unoptimised, but seems to work fine as an
unprivileged domain. You can build and install it with the
temporary make target 'make linux26', 'make install'. We'd be
interested to success reports (or otherwise).

The only issue we know of is that some glibc thread local storage
(tls) implementations don't work with it (e.g. the one shipped
with RH9). If it looks like it crashes when trying to run
/sbin/init, then try moving the /lib/tls directory out of the way
and it should fall back to /lib/i686. Please let us know if
you've had to do this with any distro other than RH9. (Debian
stable works fine).

[RH9 /lib/tls uses a positively insane scheme in which 4GB
segments are created and -ve offsets (!!) used to access within
them. We just can't allow this in Xen as it would violate
security. Hopefully other distros haven't used this particular
version. It must cause performance havoc for VMware too.  We've
been lucky with Linux 2.4 as ld.so automatically selects using
/lib/i686 for reasons I don't understand -- 'ldconfig -p'
suggests that tls should be used if "hwcap: 0x8000000000000000,
OS ABI: Linux 2.4.20". Kernel 2.4.26 is obviously > 2.4.20.
I don't know what this hwcap is. ] 

If there's anyone on the list that understands the various
different tls implementations, or how to best persuade ls.so to
ignore /lib/tls, we'd love to hear from you.

Ian


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Re: 2.6 changesets

[Keir Fraser]

> [RH9 /lib/tls uses a positively insane scheme in which 4GB
> segments are created and -ve offsets (!!) used to access within
> them. We just can't allow this in Xen as it would violate
> security. Hopefully other distros haven't used this particular
> version.

Having read up on it, NPTL is the new ABI for thread-local storage as
concocted by Ulrich Drepper and Redhat. Given that Ulrich is the glibc
maintainer, it seems likely to get pushed as the default config,
unless other distros deliberately unconfigure it.

> It must cause performance havoc for VMware too.  We've
> been lucky with Linux 2.4 as ld.so automatically selects using
> /lib/i686 for reasons I don't understand -- 'ldconfig -p'
> suggests that tls should be used if "hwcap: 0x8000000000000000,
> OS ABI: Linux 2.4.20". Kernel 2.4.26 is obviously > 2.4.20.
> I don't know what this hwcap is. ] 


Actually I see 2.4.26 going for /lib/libc lib/libm and so on, rather
than /lib/i686/*. Not sure why that is, since 'uname' says it's a i686
platform. 

 -- Keir
 
> If there's anyone on the list that understands the various
> different tls implementations, or how to best persuade ls.so to
> ignore /lib/tls, we'd love to hear from you.
> 
> Ian
> 
> 
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Re: 2.6 changesets

[Nuno Silva]

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Hi, Ian!

Please read below:

Ian Pratt wrote:

[..]

|
| If there's anyone on the list that understands the various
| different tls implementations, or how to best persuade ls.so to
| ignore /lib/tls, we'd love to hear from you.
|

I'm not an expert or anything like that but I investigated this same
problem because UML (user mode linux) doesn't like (read: support) tls
too - either 2.4 and 2.6 umls.

"various different tls implementations" -> No such thing. NPTL/TLS
support is stable and much the same in every linux distro. So if you
have problems with RH you'll have problems with debian unstable, i686
and a 2.6 kernel (read below for explanation), etc...

/lib/ld.so checks `uname` in 2.4 kernels. If extraversion begins with
"-ntpl" than this 2.4 kernel has nptl support. If extraversion doesn't
have that string it's assumed that the kernel doesn't have nptl support
(and tls support).

If the kernel is 2.6+ then ld.so assumes that nptl support is present.
The way to remove it is to "mv /lib/tls /lib/tls.off". Other way to deal
with it is to use the environment var LD_ASSUME_KERNEL and set it to 2.4
globally, wich may cause havoc :)

I think that nptl requires at least i486, so if you install for i386,
nptl support won't be included in the installation. Another way is to
downgrade the VMM from i686 (or whatever processor you have) to plain
i386. I don't know if it's possible in xen...

In my uml setups i choose the KISS method: mv /lib/tls /lib/tls.off

With debian you can instruct dpkg (man dpkg-divert) to relocate
everything that would be installed to /lib/tls to another location
automagically. This saves troubles when apt-get upgrad'ing ;)

Hope this helps,
Nuno Silva

P.S. Great work! Keep going!! :)
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Re: 2.6 changesets

[Keir Fraser]

More info on the TLS problems. I've added limited instruction
emulation to Xen so that we can use the TLS libraries that come with
e.g., Redhat 9 and Debian. Basically we take teh general protection
fault, then decode the instruction and interpret it if it's trying to
access a -ve segment offset.

Unfortunately the emulation triggers _a lot_! So it will affect
performance...

However, I intend to add dynamic patching of the unexecutable
instructions to Linux -- it will detect instructions of a particular
limited class, and dynamically create fixup code, and patch the
original execution site to call the fixup code.

I originally thought this would be very hard, but I think I have a way
of doing this in a reasonably simple manner (fingers crossed). This
should get almost every drop of performance back.

 -- Keir

> I'm not an expert or anything like that but I investigated this same
> problem because UML (user mode linux) doesn't like (read: support) tls
> too - either 2.4 and 2.6 umls.
> 
> "various different tls implementations" -> No such thing. NPTL/TLS
> support is stable and much the same in every linux distro. So if you
> have problems with RH you'll have problems with debian unstable, i686
> and a 2.6 kernel (read below for explanation), etc...
> 
> /lib/ld.so checks `uname` in 2.4 kernels. If extraversion begins with
> "-ntpl" than this 2.4 kernel has nptl support. If extraversion doesn't
> have that string it's assumed that the kernel doesn't have nptl support
> (and tls support).
> 
> If the kernel is 2.6+ then ld.so assumes that nptl support is present.
> The way to remove it is to "mv /lib/tls /lib/tls.off". Other way to deal
> with it is to use the environment var LD_ASSUME_KERNEL and set it to 2.4
> globally, wich may cause havoc :)
> 
> I think that nptl requires at least i486, so if you install for i386,
> nptl support won't be included in the installation. Another way is to
> downgrade the VMM from i686 (or whatever processor you have) to plain
> i386. I don't know if it's possible in xen...
> 
> In my uml setups i choose the KISS method: mv /lib/tls /lib/tls.off
> 
> With debian you can instruct dpkg (man dpkg-divert) to relocate
> everything that would be installed to /lib/tls to another location
> automagically. This saves troubles when apt-get upgrad'ing ;)
> 
> Hope this helps,
> Nuno Silva
> 
> P.S. Great work! Keep going!! :)
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.2.4 (GNU/Linux)
> Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org
> 
> iD8DBQFA+KHUOPig54MP17wRAmQ6AKDJt+S34QrrzXN0Bm4hFlBkNpW4dgCgw55G
> EFOTQDye2YnPA/foE9R0OUs=
> =L05g
> -----END PGP SIGNATURE-----
> 
> 
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> FREE Java Enterprise J2EE developer tools!
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> _______________________________________________
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> Xen-devel@lists.sourceforge.net
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