Re: nVMX regression v3.13+, bisected

[Kashyap Chamarthy <kchamart@redhat.com>]

On Thu, Feb 27, 2014 at 02:14:23AM +0530, Kashyap Chamarthy wrote:
> On Wed, Feb 26, 2014 at 09:27:17PM +0100, Stefan Bader wrote:
> > On 26.02.2014 21:25, Paolo Bonzini wrote:
> 
> [. . .]
> 
> > >>
> > >> I bisected this and ended up on the following commit which, when reverted made
> > >> the launch work again:
> > >>
> > >> Author: Anthoine Bourgeois <bourgeois@bertin.fr>
> > >> Date:   Wed Nov 13 11:45:37 2013 +0100
> > >>
> > >>     kvm, vmx: Fix lazy FPU on nested guest
> > >>
> > >>     If a nested guest does a NM fault but its CR0 doesn't contain the TS
> > >>     flag (because it was already cleared by the guest with L1 aid) then we
> > >>     have to activate FPU ourselves in L0 and then continue to L2. If TS flag
> > >>     is set then we fallback on the previous behavior, forward the fault to
> > >>     L1 if it asked for.
> > >>
> > >>     Signed-off-by: Anthoine Bourgeois <bourgeois@bertin.fr>
> > >>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > >>
> > >> The condition to exit to L0 seems to be according to what the description says.
> > >> Could it be that the handling in L0 is doing something wrong?
> > > 
> > > Thanks, I'll look at it tomorrow or Friday.
> > > 
> > > Paolo
> > > 
> > Great thanks. And maybe it helps if I actually add the link to the bug report as
> > I had intended... :-P
> > 
> > [1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1278531
> 
> Yes, I'm seeing something similar[*] in a consistent manner with minimal
> Fedora installs on L0, L1 and L2
 
Ok, I just tried to debug an L2 guest (a libguestfs appliance) via
gdb following this method[1]. This is how far I got:


>From shell on L1, launch the libguestfs appliance (note: here libguestfs is
compiled with gdb debugging enabled, so QEMU won't start running the
appliance):

    $ ./run libguestfs-test-tool
    [. . .]
    checking modpath /lib/modules/3.14.0-0.rc2.git0.1.fc21.x86_64 is a directory
    picked kernel vmlinuz-3.14.0-0.rc2.git0.1.fc21.x86_64
    supermin helper [00000ms] finished creating kernel
    [. . .]
    libguestfs: warning: qemu debugging is enabled, connect gdb to tcp::1234 to begin
    [. . .]


>From a different shell, I invoke gdb like that:


    (gdb) symbol-file  /usr/lib/debug/lib/modules/3.14.0-0.rc4.git0.1.fc21.x86_64/vmlinux 
    Reading symbols from /usr/lib/debug/lib/modules/3.14.0-0.rc4.git0.1.fc21.x86_64/vmlinux...done.
    (gdb) target remote tcp::1234
    Remote debugging using tcp::1234
    0x0000fff0 in ftrace_stack ()
    (gdb) bt
    #0  0x00000997 in irq_stack_union ()
    #1  0x00000000 in ?? ()
    (gdb) 
    (gdb) c
    Continuing.


Again, back to libguestfs-test-tool, it's just hung attempting to booting from ROM:

    [. . .]
    SGABIOS $Id: sgabios.S 8 2010-04-22 00:03:40Z nlaredo $ (mockbuild@) Wed Aug 14 23:57:08 UTC 2013
    Term: 80x24
    4 0
    SeaBIOS (version 1.7.4-20140106_154858-)
    Booting from ROM...


Back to gdb, to find out _what_ file the above function is trying to be
executed from:


    (gdb) c
    Continuing.
    ^C
    Program received signal SIGINT, Interrupt.
    0x00000997 in irq_stack_union ()
    (gdb) bt
    #0  0x00000997 in irq_stack_union ()
    #1  0x00000000 in ?? ()
    (gdb) list
    1       /*
    2        * Copyright 2002, 2003 Andi Kleen, SuSE Labs.
    3        *
    4        * This file is subject to the terms and conditions of the GNU General Public
    5        * License.  See the file COPYING in the main directory of this archive
    6        * for more details. No warranty for anything given at all.
    7        */
    8       #include <linux/linkage.h>
    9       #include <asm/dwarf2.h>
    10      #include <asm/errno.h>
    (gdb) 
    [. . .]
    (gdb) 
    241     ENDPROC(csum_partial_copy_generic)
    (gdb) 
    Line number 242 out of range; arch/x86/lib/csum-copy_64.S has 241 lines.
    (gdb)


PS: Paolo, I'll try to test with your new patch soon.

Thanks.

  [1] https://github.com/libguestfs/libguestfs/blob/master/src/launch-direct.c#L404


> 
>   [*] https://bugzilla.kernel.org/show_bug.cgi?id=69491#c7
> 
> 

-- 
/kashyap