Date: Thu, 28 Feb 2008 10:40:00 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Klaus S. Madsen" <ksm@hjernemadsen.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Pavel Machek <pavel@ucw.cz>,
       "Rafael J. Wysocki" <rjw@sisk.pl>, "H. Peter Anvin" <hpa@zytor.com>,
       Thomas Gleixner <tglx@linutronix.de>
Subject: Re: Regression in 2.6.25-rc3: s2ram segfaults before suspending
Message-ID: <20080228094000.GA2987@elte.hu>
References: <20080227221033.GR17932@hjernemadsen.org> <20080228091639.GB1133@elte.hu> <20080228092846.GC17932@hjernemadsen.org>
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* Klaus S. Madsen <ksm@hjernemadsen.org> wrote:

> > > 524             int r;
> > > 525     #ifdef __PIC__
> > > 526             asm volatile (
> > > 527              "pushl %%ebx\n\t"
> > > 528              "movl %2, %%ebx\n\t"
> > > 529              "int $0x80\n\t"
> > > 530              "popl %%ebx"
> > > (gdb) bt 
> > > #0  0xb7facf4a in run_vm86 () at lrmi.c:526
> > > #1  0xb7fad61b in LRMI_int (i=16, r=0xbffca670) at lrmi.c:844
> > > #2  0x0804acfc in do_vbe_service (AX=20227, BX=0, regs=0xbffca670)
> > >     at vbetool/vbetool.c:158
> > > #3  0x0804af7e in __get_mode () at vbetool/vbetool.c:453
> > > #4  0x0804a30f in s2ram_hacks () at s2ram-x86.c:268
> > > #5  0x0804954f in main (argc=1, argv=0x0) at s2ram-main.c:92
> > > 
> > > I have tried to bisect the problem, and it fingered the following
> > > commit:
> > > 
> > > commit 82bc03fc158e28c90d7ed9919410776039cb4e14
> > > Author: Ingo Molnar <mingo@elte.hu>
> > > 
> > >     x86: add PWT to NOCACHE flags
> > > 
> > > Reverting this commit in the bisected tree (by executing git show 
> > > 82bc03fc158e28c90d7ed9919410776039cb4e14 | patch -R -p1), makes the 
> > > segfault go away. I've run make clean between each kernel compile, to 
> > > be sure the tree was correctly compiled.
> > 
> > thanks for tracking this down. It would be nice to figure out why this 
> > change made a difference. Perhaps VM86 mode has some restrictions in 
> > what type of pagetables it can operate in - and the CPU just refuses to 
> > properly emulate those 16-bit instructions? (this would be very weird). 
> > We are trying to execute 16-bit BIOS code here, right?
> > 
> > which instruction is the segfault coming from - the int $0x80? So in 
> > vm86 mode we generated a #GPF which shows up as a SIGSEGV?
> I must say, that I don't quite understand why gdb fingers the "asm
> volatile" line and not one of the assembly lines, when reporting the
> segfault. But I'm not really well versed in lowlevel gdb use, so if you
> could give me a about how I get gdb to disassemble the code at the
> instruction pointer, I'll return with the result.

typing 'disassemble' should do the trick.

If you have a specific address outside of the current instruction 
pointer, then doing disassembly on a range:

  disassemble 0xb7facf4a 0xb7facf8a

should work too.

	Ingo
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