[Crash-utility] [PATCH] bug on get_be_long() and improvement of bt

[Dave Anderson]

----- "Hu Tao" <hutao at cn.fujitsu.com> wrote:

> On Tue, Oct 19, 2010 at 09:06:33AM -0400, Dave Anderson wrote:
> > 
> > ----- "Hu Tao" <hutao at cn.fujitsu.com> wrote:
> > 
> > > Hi Dave,
> > > 
> > >    These are updated patches tested with SMP system and panic
> task.
> > > 
> > >    When testing a x86 guest, I found another bug about reading cpu
> > >    registers from dumpfile. Qemu simulated system is x86_64 
> > >    (qemu-system-x86_64), guest OS is x86. When crash reads cpu registers
> > >    from dumpfile, it uses cpu_load_32(), this will read gp registers by
> > >    get_be_long(fp, 32), that is, treate them as 32bits. But in fact,
> > >    qemu-system-x86_64 saves 64bits for each of them(although guest OS
> > >    uses only lower 32 bits). As a result, crash gets wrong cpu gp
> > >    register values.
> > 
> > As I understand it, you're running a 32-bit guest on a 64-bit host.
> 
> Yes.
> 
> > If you were to read 64-bit register values instead of 32-bit register
> > values, wouldn't that cause the file offsets of the subsequent get_xxx()
> > calls in cpu_load() to read from the wrong file offsets?  And then
> > that would leave the ending file offset incorrect, such that the 
> > qemu_load() loop would fail to find the next device?
> > 
> > In other words, the cpu_load() function, which is used for both
> > 32-bit and 64-bit guests, must be reading the correct amount of
> > data from the "cpu" device, or else qemu_load() would fail to
> > find the next device in the next location in the dumpfile.  
> 
> True. In fact, in my case if read 32-bit registers, following devices
> are found:
>     block, ram, kvm-tpr-opt, kvmclock, timer, cpu_common, cpu.
> If read 64-bit registers, following devices are found:
>     block, ram, kvm-tpr-opt, kvmclock, timer, cpu_common, cpu, apic, fw_cfg

Right -- so it got "lost" after incorrectly gathering the data for the
first "cpu" device instance.

> > >    Is there any way we can know from dumpfile that these gp
> > >    registers(and those similar registers) are 32bits or 64bits?
> > 
> > I don't know.  If what you say is true, when would those registers
> > ever be 32-bit values?
> 
> I did tests on a 64-bit machine. Result is:
> 
> machine		OS	guest machine		guest OS	saved gp regs
> ------------------------------------------------------------------------
> 64-bit		x86	qemu-kvm(kvm enabled)	x86		64 bits
> 64-bit		x86	qemu(kvm disabled)	x86		32 bits

I don't understand what you mean when you say that the guest machine
is "kvm enabled" or "kvm disabled"?

And if your host machine is running a 32-bit x86 OS (on 64-bit hardware),
that's something I've never seen given that Red Hat only allows 64-bit
kernels as KVM hosts.

Dave