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

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Message-ID: <1391933453.1014.20.camel@wall-e.seibold.net>
Date:	Sun, 09 Feb 2014 09:10:53 +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 Samstag, den 08.02.2014, 21:22 -0800 schrieb Andi Kleen:
> > +notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
> > +{
> > +	long ret;
> > +
> > +	asm(
> > +		"push %%ebx \n"
> > +		"mov %2,%%ebx \n"
> > +		"call VDSO32_vsyscall \n"
> > +		"pop %%ebx \n"
> > +		: "=a" (ret)
> > +		: "0" (__NR_clock_gettime), "d" (clock), "c" (ts)
> > +		: "memory");
> 
> The push/mov/pop could be just replaced with "b"?

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'".

> 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. 

> Same in the other sys calls.
> 

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. 

> > @@ -310,6 +311,11 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
> >  	unsigned long addr;
> >  	int ret = 0;
> >  	bool compat;
> > +	struct vm_area_struct *vma;
> > +	extern char __vvar_page;
> > +#ifdef CONFIG_HPET_TIMER
> > +	extern unsigned long hpet_address;
> > +#endif
> 
> Put this into a header.
> 

Okay...

> > +++ b/arch/x86/vdso/vdso32/vclock_gettime.c
> > @@ -0,0 +1,16 @@
> > +#define BUILD_VDSO32
> > +
> > +#ifdef CONFIG_X86_64
> > +
> > +#define _ASM_X86_PAGE_H
> > +
> > +#define __pa(x)                0
> > +#define __va(x)                0
> > +
> > +#undef CONFIG_ILLEGAL_POINTER_VALUE
> > +#define CONFIG_ILLEGAL_POINTER_VALUE   0
> 
> This needs some comments.
> 
> Why are the __pa/__va defines safe?
> 
> The rest of the patchkit looks reasonable to me from a quick read.
> Much better than the first version.
> 

This is due the -m32 compilation, i will get a log of 
"warning: integer constant is too large for 'unsigned long' type",
because a unsigned long is only 32 bit.

Since the vclocktime.c use a lot of kernel header a little bit hacking
is necessary, because the CONFIG_X86_64 will attract a lot of x86-64
headers, which are not compatible with 32 bit compilation. 

The _ASM_X86_PAGE_H prevents the include of arch/x86/include/asm/page.h.

But the inline function phys_to_virt() in arch/x86/include/asm/io.h
depends on the __pa() and __va() functions, which comes from
arch/x86/include/asm/page.h. So i need two dummys for this.

Since non of this is functions is used in arch/x86/vdso/vclock_gettime.c
it is save.

I have no idea how to fix this weaved include file web without change a
lot in many include files.

The define of CONFIG_ILLEGAL_POINTER_VALUE is also to prevent the
""warning: integer constant is too large...".

- Stefani

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