Linux applications core if running (k)qemu

[Kostik Belousov]

On Mon, Sep 08, 2008 at 10:34:23PM +0200, Juergen Lock wrote:
> On Mon, Sep 08, 2008 at 12:53:00AM +0300, Kostik Belousov wrote:
> > On Sun, Sep 07, 2008 at 12:15:06AM +0200, Juergen Lock wrote:
> > > In article <20080906152929.GB2038 at deviant.kiev.zoral.com.ua> you write:
> > > >-=-=-=-=-=-
> > > >
> > > >On Sat, Sep 06, 2008 at 02:46:59PM +0400, Chagin Dmitry wrote:
> > > >> On Tue, Sep 02, 2008 at 03:56:33PM -0500, Sean C. Farley wrote:
> > > >> > On Sat, 30 Aug 2008, Chagin Dmitry wrote:
> > > >> > 
> > > >> > >On Fri, Aug 29, 2008 at 05:29:09PM -0500, Sean C. Farley wrote:
> > > >> > >>I am having trouble with kqemu.ko and linux.ko.  If I run qemu with
> > > >> > >>the following command, Linux applications (chroot, acroread, ls) will
> > > >> > >>start core dumping:
> > > >> > >>    qemu-system-x86_64 -m 512 \
> > > >> > >>    -drive file=/usr/QEMU/WinXP/c.img,if=ide,media=disk -boot c \
> > > >> > >>    -std-vga -parallel none -serial none -monitor stdio \
> > > >> > >>    -net nic,model=e1000 -net tap,ifname=tap0,script=no -localtime
> > > >> > >>
> > > >> > >>Loading kqemu.ko does not cause the problem, but the cores start a
> > > >> > >>little after WinXP starts running.  Unloading kqemu.ko does not help;
> > > >> > >>the cores still happen but more randomly.  I even tried unloading all
> > > >> > >>linux modules and reloading them without luck.  It takes a reboot.
> > > >> > >>
> > > >> > >>Packages:
> > > >> > >>qemu-devel-0.9.1s.20080620_1
> > > >> > >>kqemu-kmod-devel-1.4.0.p1
> > > >> > >>linux_base-f8-8_4
> > > >> > >>
> > > >> > >>sysctl:
> > > >> > >>compat.linux.osrelease: 2.6.16
> > > >> > >>
> > > >> > >>dmesg:
> > > >> > >>kqemu version 0x00010400
> > > >> > >>kqemu: KQEMU installed, max_locked_mem=1792492kB.
> > > >> > >>
> > > >> > >>System is 7-STABLE as of r181963 with or without the patch to fix RT
> > > >> > >>signals from Chagin.
> > > >> > >
> > > >> > >Interestingly... Sean, can you provide ktrace/kdump log of coring
> > > >> > >apps?  thnx!
> > > >> > 
> > > >> > Here they are (good and bad):
> > > >> > http://www.farley.org/freebsd/tmp/linuxulator_vs_kqemu/
> > > >> > 
> > > >> > The good trace is after the bad trace.  I just kept running ktrace
> > > >> > /compat/linux/bin/date over and over until I got a good trace.  Before
> > > >> > loading kqemu and running qemu, there were no core dumps.  Also, I
> > > >> > compared two bad traces and they were basically the same except for PID
> > > >> > and a couple of addresses (still very close in value).
> > > >> > 
> > > >> 
> > > >> Most likely it is a tls problem again, some days ago kib@ has made MFC
> > > >> r182684, probably it will help..
> > > >
> > > >I doubt it. This seems to be an ingenious kqemu bug. As far as I remember,
> > > >it tries to use GDT/LDT. This probably has unwanted interaction with
> > > >PCB_GS32BIT.
> > > 
> > > Wow.  That corner of the code had escaped me so far, and yes this (in
> > > amd64/linux32) looks like it won't like kqemu's seperating of the gdts
> > > on SMP indeed.  (it stores a pointer to &gdt[GUGS32_SEL] in pcb_gs32p and
> > > lets linux processes manipulate the segment pointed to by it, and when
> > > kqemu is (or was) running this won't be used by all cpus, see older threads
> > > like
> > > 	http://lists.freebsd.org/pipermail/freebsd-emulation/2008-May/004902.html
> > > for the reasons.)
> > > 
> > >  What I wonder tho is, won't this also cause problems without kqemu when
> > > there are linux processes running on multiple cpus that manipulate this
> > > segment because the gdt is then shared between the cpus?  (like, linux
> > > process on cpu 0 changes the segment, then linux process on cpu 1 comes
> > > along and changes it again and then the linux process on cpu 0 will pick
> > > it up from cpu 1?)  At least I must have somehow assumed the shared gdt
> > > wouldn't be changed later because of reasons like this...
> > 
> > Very nice catch! Me and Peter Wemm discussed the right approach,
> > that consists of actually providing per-cpu GDT. Patch is at
> > http://people.freebsd.org/~kib/misc/amd64_gdt.1.patch
> > 
> > Please, test and give a feedback. Even reports about thinks working
> > the same as before the patch are important.
> 
> OK I just tested the patch on RELENG_7 (updated my amd64 SMP box from
> RELENG_7_0) and found no problems.  (I tested linux date(1), googleearth,
> kqemu, and a few other non-linux things so far.)

Thank you for the testing. Patch was committed to HEAD already (separated
into four mostly self-contained commits).
I expect the MFC in one week, your testing is important for MFC decision.
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