linux-kernel - Re: [PATCH v12 9/9] Add 32 bit VDSO time support for 64 bit kernel

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Date:	Wed, 5 Feb 2014 14:01:02 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	andriy.shevchenko@...ux.intel.com, Martin.Runge@...de-schwarz.com,
	Andreas.Brief@...de-schwarz.com
Subject: Re: [PATCH v12 9/9] Add 32 bit VDSO time support for 64 bit kernel

On Wed, Feb 5, 2014 at 12:20 AM,  <stefani@...bold.net> wrote:
> From: Stefani Seibold <stefani@...bold.net>
>
> This patch add the VDSO time support for the IA32 Emulation Layer.
>
> Due the nature of the kernel headers and the LP64 compiler where the
> size of a long and a pointer differs against a 32 bit compiler, there
> is some type hacking necessary.
>
> The vsyscall_gtod_data struture must be a little bit rearranged, to
> serve 32- and 64-bit code access:
>
> - The seqcount_t was replaced by an unsigned, this makes the
>   vsyscall_gtod_data intedepend of kernel configuration and internal functions.
> - The structure is now packed, so it can accessed from 32- und 64- bit
>   code at the same time.
> - The inner struct clock was removed, to make packing of the while
>   struct easier.
>
> The "unsigned seq" would be handled by functions derivated from seqcount_t.
>
> Signed-off-by: Stefani Seibold <stefani@...bold.net>
> ---
>  arch/x86/include/asm/vgtod.h          |  20 +++---
>  arch/x86/kernel/vsyscall_gtod.c       |  26 +++++--
>  arch/x86/vdso/vclock_gettime.c        | 129 ++++++++++++++++++++++++----------
>  arch/x86/vdso/vdso32/vclock_gettime.c |  11 +++
>  include/uapi/linux/time.h             |   2 +-
>  5 files changed, 132 insertions(+), 56 deletions(-)
>
> diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
> index 46e24d3..2567b02 100644
> --- a/arch/x86/include/asm/vgtod.h
> +++ b/arch/x86/include/asm/vgtod.h
> @@ -4,16 +4,18 @@
>  #include <asm/vsyscall.h>
>  #include <linux/clocksource.h>
>
> -struct vsyscall_gtod_data {
> -       seqcount_t      seq;
> +/*
> + * vsyscall_gtod_data will be accessed by 32 and 64 bit code at the same time
> + * so the structure must be packed
> + */
> +struct __attribute__((packed)) vsyscall_gtod_data {
> +       unsigned seq;

Is that actually true?  At least in the part you're changing,
everything looks like it's aligned correctly.

It's almost certainly having some kind of BUILD_BUG_ON that will catch
the case where this structure's size changes.  I suspect that some
kind of asm-offsets magic can be used for this.

>
> -       struct { /* extract of a clocksource struct */
> -               int vclock_mode;
> -               cycle_t cycle_last;
> -               cycle_t mask;
> -               u32     mult;
> -               u32     shift;
> -       } clock;
> +       int vclock_mode;
> +       cycle_t cycle_last;
> +       cycle_t mask;
> +       u32     mult;
> +       u32     shift;
>
>         /* open coded 'struct timespec' */
>         time_t          wall_time_sec;
> diff --git a/arch/x86/kernel/vsyscall_gtod.c b/arch/x86/kernel/vsyscall_gtod.c
> index 91862a4..ca48248 100644
> --- a/arch/x86/kernel/vsyscall_gtod.c
> +++ b/arch/x86/kernel/vsyscall_gtod.c
> @@ -16,6 +16,18 @@
>
>  DEFINE_VVAR(struct vsyscall_gtod_data, vsyscall_gtod_data);
>
> +static inline void gtod_write_begin(unsigned *s)
> +{
> +       ++*s;
> +       smp_wmb();
> +}
> +
> +static inline void gtod_write_end(unsigned *s)
> +{
> +       smp_wmb();
> +       ++*s;
> +}
> +

Someone else should probably comment on the style for this.  Maybe
this should live in a header somewhere.  IMO if it's called
gtod_write_begin, it should take a pointer to gtod as a parameter.


> +struct api_timeval {
> +       long    tv_sec;         /* seconds */
> +       long    tv_usec;        /* microseconds */
> +};
> +
> +struct api_timespec {
> +       long    tv_sec;         /* seconds */
> +       long    tv_nsec;        /* nanoseconds */
> +};

Did you address my question about why there are two versions of this?
Shouldn't it just match the userspace headers regardless of what the
host kernel is?

Minor nit: this should be abi_time... instead of api.


You have *still* not responded to my objection to the unconditional fixmaps.

--Andy
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