Re: "hwcap 0 nosegneg" doesnt work with paravirt_ops xen as of 220.127.116.11
From: Jeremy Fitzhardinge
Date: Wed Nov 28 2007 - 13:12:21 EST
Andres Freund wrote:
> after converting a host from using a ubuntu patched 2.6.22 domU to using
> 2.6.23 xen via paravirt_ops (no configuration change except devicename change
> in fstab, and another getty running) ldconfig no longer picks up the nosegneg
> libc leading to segfaults for some programs.
It shouldn't ever lead to segfaults. At worst, you should get poor
performance because the hypervisor emulates the -ve segment offset.
> hwcap 0 nosegneg
This looks OK. If this doesn't work, then it suggests there's something
wrong with the kernel vdso which is not exporting the nonegseg
(pseudo-)hardware capability. But we've gone over this several times
now, and I was fairly sure we'd finally got it right - works for me, at
The alternative is that there's some bug on the userspace side in your
version of Ubuntu...
> On ubuntu's 2.6.22-xen:
> ldd /usr/bin/perl
> linux-gate.so.1 => (0xf57fe000)
> libdl.so.2 => /lib/tls/i686/nosegneg/libdl.so.2 (0xb7f90000)
> libm.so.6 => /lib/tls/i686/nosegneg/libm.so.6 (0xb7f6b000)
> libpthread.so.0 => /lib/tls/i686/nosegneg/libpthread.so.0 (0xb7f53000)
> libc.so.6 => /lib/tls/i686/nosegneg/libc.so.6 (0xb7e05000)
> libcrypt.so.1 => /lib/tls/i686/nosegneg/libcrypt.so.1 (0xb7dd7000)
> /lib/ld-linux.so.2 (0xb7f9d000)
> On selfcompiled 18.104.22.168:
> ldd /usr/bin/perl
> linux-gate.so.1 => (0xf57fc000)
> libdl.so.2 => /lib/tls/i686/cmov/libdl.so.2 (0xb7f3b000)
> libm.so.6 => /lib/tls/i686/cmov/libm.so.6 (0xb7f16000)
> libpthread.so.0 => /lib/tls/i686/cmov/libpthread.so.0 (0xb7efe000)
> libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb7db3000)
> libcrypt.so.1 => /lib/tls/i686/cmov/libcrypt.so.1 (0xb7d85000)
> /lib/ld-linux.so.2 (0xb7f46000)
> I now discover, that after some playing around (during discovering what the
> problem is) i must have changed something, because ldd still shows the use of
> the i686/cmov libraries, but applications reliably causing crashes earlier,
> dont do so anymore.
As I said, you shouldn't be seeing crashes either way. If you are, it
suggests a hypervisor bug. Does "xm dmesg" show anything?
> Perhaps Im all wrong, and paravirt_ops doesnt need the nosegneg versions and
> just a ldconfig or so (which seems strange because the result before and
> after the ldconfig were the same).
nosegneg is still necessary, and it should still be working as it did
> PS: I did this conversion mainly, because I was getting quite random crashes
> inside the domU. I guess, as this was a vendor kernel + xensource xen kernel,
> this isnt appropriate for lkml? As well as the crashes of the Dom0 (ubuntu
Yeah, anything which isn't 2.6.18-xen isn't officially "Xen project",
but its worth reporting the bugs to xen-devel (and Ubuntu, of course).
And to me, for that matter. I won't be able to help directly, but the
crashes may highlight things we need to be careful about in the pvops
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