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Message-ID: <1355259259.30831.13.camel@wall-e>
Date: Tue, 11 Dec 2012 21:54:19 +0100
From: Stefani Seibold <stefani@...bold.net>
To: John Stultz <john.stultz@...aro.org>
Cc: linux-kernel@...r.kernel.org, ak@...ux.intel.com,
aarcange@...hat.com, luto@...capital.net
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel
Am Dienstag, den 11.12.2012, 11:27 -0800 schrieb John Stultz:
> On 12/11/2012 08:11 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.
> 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.
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