linux-kernel - Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

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Message-ID: <1355258415.30831.2.camel@wall-e>
Date:	Tue, 11 Dec 2012 21:40:15 +0100
From:	Stefani Seibold <stefani@...bold.net>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-kernel@...r.kernel.org, ak@...ux.intel.com,
	aarcange@...hat.com, john.stultz@...aro.org
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

Am Dienstag, den 11.12.2012, 11:37 -0800 schrieb Andy Lutomirski:
> On Tue, Dec 11, 2012 at 8:11 AM,  <stefani@...bold.net> wrote:
> > From: Stefani Seibold <stefani@...bold.net>
> >
> > This small patch add the functions vdso_gettimeofday(), vdso_clock_gettime()
> > and vdso_time() support to the VDSO for x86 32-bit kernels.
> >
> > The reason to do this was to get a fast reliable time stamp. Many developers
> > uses TSC to get a fast time time stamp, without knowing the pitfalls. VDSO
> > time functions a fast and reliable way, because the kernel knows the best time
> > source and the P- and C-state of the CPU.
> >
> > For x86 the vclock_gettime.c currently supports only the HPET and TSC timer,
> > the ACPI timer should be easily to add with an other patch.
> >
> > The helper library to use the VDSO functions can be download at
> > http://http://seibold.net/vdso.c
> 
> Wow -- another implementation.  See Documentation/vDSO/parse_vdso.c.
> (Also, I think your code will break if anyone ever strips the vdso.)
> 
> > --- a/arch/x86/vdso/vclock_gettime.c
> > +++ b/arch/x86/vdso/vclock_gettime.c
> > @@ -59,14 +59,23 @@ notrace static cycle_t vread_tsc(void)
> >
> >  static notrace cycle_t vread_hpet(void)
> >  {
> > +#ifdef CONFIG_X86_64
> >         return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
> > +#else
> > +       return readl(VVAR(vsyscall_hpet) + HPET_COUNTER);
> > +#endif
> >  }
> 
> Is 0xf0 not equal to HPET_COUNTER?
> 

Yes, but HPET_COUNTER is more readable.

> >
> >  notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
> >  {
> >         long ret;
> > +#ifdef CONFIG_X86_64
> >         asm("syscall" : "=a" (ret) :
> >             "0" (__NR_clock_gettime),"D" (clock), "S" (ts) : "memory");
> > +#else
> > +       asm("int $0x80" : "=a" (ret) :
> > +           "a" (__NR_clock_gettime), "b" (clock), "c" (ts) : "memory");
> > +#endif
> >         return ret;
> >  }
> 
> __kernel_vsyscall is probably much faster if you can figure out how to
> call it from here :)
> 

Yes i know. Thats one of my problems, because i cannot call
__kernel_vsyscall directly due the relocation. Any idea?

> >
> > @@ -74,15 +83,20 @@ notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
> >  {
> >         long ret;
> >
> > +#ifdef CONFIG_X86_64
> >         asm("syscall" : "=a" (ret) :
> >             "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
> > +#else
> > +       asm("int $0x80" : "=a" (ret) :
> > +           "a" (__NR_gettimeofday), "b" (tv), "c" (tz) : "memory");
> > +#endif
> >         return ret;
> >  }
> >
> 
> Ditto.
> 
> > --- a/arch/x86/vdso/vdso32/vdso32.lds.S
> > +++ b/arch/x86/vdso/vdso32/vdso32.lds.S
> > @@ -24,6 +24,16 @@ VERSION
> >                 __kernel_vsyscall;
> >                 __kernel_sigreturn;
> >                 __kernel_rt_sigreturn;
> > +#ifdef CONFIG_X86_32
> > +               clock_gettime;
> > +               __vdso_clock_gettime;
> > +               gettimeofday;
> > +               __vdso_gettimeofday;
> > +               gettimeofday;
> > +               __vdso_gettimeofday;
> > +               time;
> > +               __vdso_time;
> > +#endif
> 
> Please remove the non-__vdso versions.  They're not useful and x86-64
> only has them for backwards compatibility.
> 
> >         local: *;
> >         };
> >  }
> > @@ -35,3 +45,8 @@ VDSO32_PRELINK                = VDSO_PRELINK;
> >  VDSO32_vsyscall                = __kernel_vsyscall;
> >  VDSO32_sigreturn       = __kernel_sigreturn;
> >  VDSO32_rt_sigreturn    = __kernel_rt_sigreturn;
> > +#ifdef CONFIG_X86_32
> > +VDSO32_clock_gettime   = clock_gettime;
> > +VDSO32_gettimeofday    = gettimeofday;
> > +VDSO32_time            = time;
> > +#endif
> 
> Yikes.  What's this?  (Just curious.)

I don't know if anyone needs this. Maybe i can kick it away.


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