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

[Stefani Seibold <stefani@seibold.net>]

Am Dienstag, den 11.12.2012, 11:27 -0800 schrieb John Stultz:
> On 12/11/2012 08:11 AM, stefani@seibold.net wrote:
> > From: Stefani Seibold <stefani@seibold.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.
> Very cool. There have been similar implementations of this patch over 
> the years, but they were all bit more hackish then this.
> 
Thanks, i think it is not more hackish than the x86 64 code.

> 
> > 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.
> Although the ACPI PM timer requires port-io which would need tweaking to 
> allow normal users to access it. And I'm not sure if the performance 
> would be much improved, as the port-io probably dominates the 
> performance cost.
> 

It was only an idea. I think it would be easy to give the ioperm read
for the ACPI Timer without breaking anything.

> > The helper library to use the VDSO functions can be download at
> > http://http://seibold.net/vdso.c
> > The libary is very small, only 228 lines of code. Compile it with
> > gcc -Wall -O3 -fpic vdso.c -lrt -shared -o libvdso.so
> > and use it with LD_PRELOAD=<path>/libvdso.so
> >
> > This kind of helper must be integrated into glibc, for x86 64-bit and
> > PowerPC it is already there.
> >
> A few notes below...
> 
> 
> > diff --git a/arch/x86/include/asm/compat.h b/arch/x86/include/asm/compat.h
> > index 59c6c40..45ba688 100644
> > --- a/arch/x86/include/asm/compat.h
> > +++ b/arch/x86/include/asm/compat.h
> > @@ -295,6 +295,10 @@ static inline compat_uptr_t ptr_to_compat(void __user *uptr)
> >   
> >   static inline void __user *arch_compat_alloc_user_space(long len)
> >   {
> > +#ifdef CONFIG_X86_32
> > +	struct pt_regs *regs = task_pt_regs(current);
> > +	return (void __user *)regs->sp - len;
> > +#else
> >   	compat_uptr_t sp;
> >   
> >   	if (test_thread_flag(TIF_IA32)) {
> > @@ -305,6 +309,7 @@ static inline void __user *arch_compat_alloc_user_space(long len)
> >   	}
> >   
> >   	return (void __user *)round_down(sp - len, 16);
> > +#endif
> >   }
> 
> This style of in-line ifdefs are ugly and hard to read.
> 
> So instead of doing:
> void myfunction (void)
> {
> #ifdef 32bits
>      32_bit_implementation();
> #else
>      64_bit_implementation();
> #endif
> }
> 
> Where possible, please do:
> #ifdef 32bits
> void myfunction1(void)
> {
>      32_bit implementation();
> }
> void myfunction2(void)
> {
> ....
> #else /*64 bit versions */
> void myfunction1(void)
> {
>      64_bit implementation();
> }
> void myfunction2(void)
> ....
> #endif
> 
> > diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> > index 4df6c37..2dc6b72 100644
> > --- 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
> >   }
> >   
> >   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;
> >   }
> Same point here.
> 
> 

Will be fixed.

> > diff --git a/arch/x86/vdso/vdso32/vclock_gettime.c b/arch/x86/vdso/vdso32/vclock_gettime.c
> > new file mode 100644
> > index 0000000..c9a1909
> > --- /dev/null
> > +++ b/arch/x86/vdso/vdso32/vclock_gettime.c
> > @@ -0,0 +1,7 @@
> > +/*
> > + * since vgtod layout differs between X86_64 and x86_32, it is not possible to
> > + * provide a 32 bit vclock with a 64 bit kernel
> > + */
> > +#ifdef CONFIG_X86_32
> > +#include "../vclock_gettime.c"
> > +#endif
> Could you expand a bit as to why a compat layer isn't possible? It seems 
> we could easily convert the vsyscall_gtod_data to a more explicit 
> arch-neutral size. Or is it the actual data page mapping?
> 

This could be done in a subsequent patch. First i want not change to
much, to make the review easier. Converting the vsyscall_gtod_data to
arch neutral is not so easy, because the size of time_t, timezone and
timespec differs. So currently 32 bit programs running under a 64 bit
kernel will not get a VDSO with time functions. But real 32 bit kernel
will provide it.