Re: kvm_amd BUG: unable to handle kernel NULL pointer dereference at 00000014

["Roedel, Joerg" <Joerg.Roedel@amd.com>]

(CC'ing x86 Maintainers)

On Sun, Mar 06, 2011 at 05:23:12AM -0500, Avi Kivity wrote:
> On 03/04/2011 12:34 AM, IVAN ANGELOV wrote:
> > Hello,
> > This provided dmesg message and kernel behavior appear when trying to
> > run qemu-kvm with kvm_amd module. Without kvm_amd qemu-kvm runs fine
> > but a slower. I managed to see that this happens with 2.6.38-rc6 ,
> > 2.6.38-rc7 vanilla kernels compiled using kernel-package. OS ubuntu
> > natty. Using the standard toolchain and gcc from ubuntu: gcc version
> > 4.5.2 (Ubuntu/Linaro 4.5.2-3ubuntu3)
> > I reverted to 2.6.37.2 linux kernel, compiled with the very same tools
> > and machine I use qemu-kvm with kvm_amd module without any problems.
> > If I can provide some extra info about that please let me know.
> > This issue also appears with the kernel provided by the Ubuntu
> > distribution: 2.6.38-5-generic-pae #32-Ubuntu SMP Tue Feb 22 17:48:56
> > UTC 2011 i686 athlon i386 GNU/Linux , I suspect it is somehow related
> > to the 2.6.38 kernel series.
> > cpuinfo - phenom ii x4 955 mildly overclocked - 4 fields like this is
> > the whole cpuinfo.
> 
> 
>     c:    65 a1 14 00 00 00        mov    %gs:0x14,%eax
> 
> faults, gsbase == NULL.
> 
> But arch/x86/include/asm/percpu.h says:
> 
> #ifdef CONFIG_X86_64
> #define __percpu_seg        gs
> #define __percpu_mov_op        movq
> #else
> #define __percpu_seg        fs
> #define __percpu_mov_op        movl
> #endif
> 
> 
> So we should be using %fs, not %gs.

There is no access to per_cpu variables at the start of x86_decode_insn.
I did a bit of investigation and it turns out that the faulting
instruction is inserted into the code by the gcc because the
CONFIG_CC_STACKPROTECTOR is enabled.
The user tested this is Ubuntu 11.04 alpha-something i386 and this
distro uses gcc 4.5.2. So CC_STACKPROTECTOR seems to be harmful with
this gcc version but I am not sure whether this counts as a gcc bug.

	Joerg

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