linux-kernel - Re: [PATCH v13 8/9] Add 32 bit VDSO time support for 32 bit kernel

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Date:	Sun, 09 Feb 2014 12:45:17 +0100
From:	Stefani Seibold <stefani@...bold.net>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, aarcange@...hat.com, john.stultz@...aro.org,
	luto@...capital.net, xemul@...allels.com, gorcunov@...nvz.org,
	andriy.shevchenko@...ux.intel.com, Martin.Runge@...de-schwarz.com,
	Andreas.Brief@...de-schwarz.com
Subject: Re: [PATCH v13 8/9] Add 32 bit VDSO time support for 32 bit kernel

Am Sonntag, den 09.02.2014, 02:05 -0800 schrieb Andi Kleen:
> > EBX is a reserved as the base register when compiling with -fPIC.
> > Otherwise it get a "error: can't find a register in class 'BREG' while
> > reloading 'asm'".
> 
> In this case you have to dwarf annotate the push/pop
> 
> >  
> > > Also I if this calls syscall.S:__kernel_vsyscall it clobbers more registers
> > > (ecx at least?) and you need to describe that to gcc with a + 
> > 
> > Sine ECX is in use as the second parameter it is still marked as
> > clobbered. 
> 
> No input parameters are not clobbered.
> 
> > I think it should word... As i know a system call will preserve all
> > register expect the EAX, which is marked as in and out for this asm()
> > directive. 
> 
> Yes the syscall does, but the vsyscall stub does not (for the !0x80 case)
> 

I cross check all kinds of vsyscall and all do save the ECX registers. 

So IMHO i see no register modification other than EAX, which is declared
as the output register in the asm() directive.

This following is what i get with objdump -d.

./arch/x86/vdso/vdso32-sysenter.so:

__kernel_vsyscall:
push   %ecx
push   %edx
push   %ebp
mov    %esp,%ebp
sysenter 
nop
nop
nop
nop
nop
nop
nop
int    $0x80
pop    %ebp
pop    %edx
pop    %ecx
ret    

./arch/x86/vdso/vdso32-int80.so:

__kernel_vsyscall:
int    $0x80
ret

./arch/x86/vdso/vdso32-syscall.so:

__kernel_vsyscall:
push   %ebp
mov    %ecx,%ebp
syscall 
mov    $0x2b,%ecx
mov    %ecx,%ss
mov    %ebp,%ecx
pop    %ebp
ret

BTW, it is not possible to mark an input parameter with +, this can only
done for an output parameter.

If there is really an issue what i have overseen, a working solution
would be to use an other register for the second parameter (ESI or EDI)
and add ECX to the clobber list.

- Stefani


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