linux-kernel - Re: VMI broken on tip/master...

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Date:	Mon, 16 Mar 2009 15:54:26 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	akataria@...are.com
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: VMI broken on tip/master...

Alok Kataria wrote:
> On Sat, 2009-03-14 at 16:45 -0700, Jeremy Fitzhardinge wrote:
>   
>> Alok Kataria wrote:
>>     
>>> Hi Peter, 
>>>
>>> I was seeing a early fault when running tip/master with VMI enabled on
>>> VMware platform. 
>>> This early fault was in the vmi_patch code where we are applying
>>> paravirt_alternatives. After some trials i noticed that this is
>>> reproducible only with CONFIG_TRACING. With that disabled my VM boots
>>> again. 
>>>
>>> I started a git bisect after that, and git pointed to this as the bad
>>> commit
>>>
>>> commit 6cc3c6e12bb039047974ad2e7e2d46d15a1b762f
>>>     trace_clock: fix preemption bug
>>>
>>> I then reverted that commit from tip/master and the system did boot. 
>>> But I fail to understand how this simple patch would be causing things
>>> to fail in VMI. Any ideas ?
>>>   
>>>       
>> Nope.  My first guess is that this is a misbisection, but the fact that 
>> reverting helps tends to undermine that diagnosis.
>>
>> What crash are you seeing?  What kind of fault?  At what instruction?  
>>     
>
> It being a early fault, nothing is printed on the console the system
> just stays stuck in this early_fault code in arch/x86/kernel/head_32.S
>
> I don't understand why is this not printing anything at this early fault
> though, the system just enters the hlt_loop and stays there.
>   

It should drop something onto the vga console (and/or serial port?).

>> Doing what?  It's a bit hard to tell what you're actually seeing.
>>     
> I did some more debugging and I think i know what the problem is 
> The objdump for trace_clock_local looks like this
>
> c1070c24 <trace_clock_local>:
> c1070c24:       55                      push   %ebp
> c1070c25:       89 e5                   mov    %esp,%ebp
> c1070c27:       53                      push   %ebx
> c1070c28:       83 ec 08                sub    $0x8,%esp
> c1070c2b:       ff 15 5c 87 3d c1       call   *0xc13d875c
> c1070c31:       ba 64 87 3d c1          mov    $0xc13d8764,%edx
> c1070c36:       89 45 f4                mov    %eax,0xfffffff4(%ebp)
> c1070c39:       ff 12                   call   *(%edx)		<<=====*
> c1070c3b:       e8 cd 68 f9 ff          call   c100750d <sched_clock>
> c1070c40:       89 c1                   mov    %eax,%ecx
> c1070c42:       8b 45 f4                mov    0xfffffff4(%ebp),%eax
> c1070c45:       ff 15 60 87 3d c1       call   *0xc13d8760
>
> Notice instruction on c1070c39 we have "call *(edx)",
> edx was just loaded with the address for the paravirt call.
> when we try to replace that to a call to vmi specific function, maybe we
> hit a BUG_ON(len < 5) in vmi's patch_internal code, because now the
> instruction length is less than 5.
>
> Is there is a way to get GCC to not do such fancy tricks, and instead do
> a direct "call 0xc13d8764"  ?
Well, indirect "call *0xc13d875c".   But yes, its hard to see what gcc 
thinks its getting out of doing the indirect call via %edx.  We 
definitely don't want gcc doing such things, even if it does make sense 
(such as calling an op multiple times, and CSEing the pointer); given 
that we generate all this in asm anyway, we could force it.  Does this help?

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index d4fec1f..62dfc51 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -395,7 +395,7 @@ extern struct pv_lock_ops pv_lock_ops;
 
 #define paravirt_type(op)				\
 	[paravirt_typenum] "i" (PARAVIRT_PATCH(op)),	\
-	[paravirt_opptr] "m" (op)
+	[paravirt_opptr] "i" (&(op))
 #define paravirt_clobber(clobber)		\
 	[paravirt_clobber] "i" (clobber)
 
@@ -449,7 +449,7 @@ int paravirt_disable_iospace(void);
  * offset into the paravirt_patch_template structure, and can therefore be
  * freely converted back into a structure offset.
  */
-#define PARAVIRT_CALL	"call *%[paravirt_opptr];"
+#define PARAVIRT_CALL	"call *%c[paravirt_opptr];"
 
 /*
  * These macros are intended to wrap calls through one of the paravirt


    J
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