Re: [PATCH 4/4] Add 32 bit VDSO support for 64 kernel

[Stefani Seibold <stefani@seibold.net>]

Am Donnerstag, den 30.01.2014, 10:21 -0800 schrieb Andy Lutomirski:
> On Thu, Jan 30, 2014 at 2:49 AM,  <stefani@seibold.net> wrote:
> > From: Stefani Seibold <stefani@seibold.net>
> >
> > This patch add the support for the IA32 Emulation Layer to run 32 bit
> > applications on a 64 bit kernel.
> >
> > 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 a lot of type hacking necessary.
> >
> > This kind of type hacking could be prevent in the future by doing a call to the
> > 64 bit code by the following sequence:
> >
> > - Compile the arch/x86/vdso/vclock_gettime.c as 64 bit, but only generate
> >   the assemble output.
> > - Next compile a 32 bit object by including the 64 bit vclock_gettime.s
> >   prefixed with .code64
> > - At least we need a trampolin code which invokes the 64 bit code and do
> >   the API conversation (64 bit longs to 32 bit longs), like the
> >   followig snipped:
> >
> > ENTRY(call64)
> >         push %ebp
> >         movl %esp, %ebp
> >         ljmp $__USER_CS, $1f
> > .code64
> 
> I bet that this trampoline takes at least as long as a syscall /
> sysenter instruction.  I'd be surprised if designers of modern cpus
> care at all about ljmp latency.
> 

I have no idea, this must be measured. The code is smaller and it would
save a lot of compaility issues.

> 
> >
> > Signed-off-by: Stefani Seibold <stefani@seibold.net>
> > ---
> >  arch/x86/vdso/vclock_gettime.c        | 112 ++++++++++++++++++++++++++--------
> >  arch/x86/vdso/vdso32/vclock_gettime.c |   7 +++
> >  2 files changed, 95 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> > index 19b2a49..a2417e2 100644
> > --- a/arch/x86/vdso/vclock_gettime.c
> > +++ b/arch/x86/vdso/vclock_gettime.c
> > @@ -31,12 +31,24 @@
> >
> >  #define gtod (&VVAR(vsyscall_gtod_data))
> >
> > +struct api_timeval {
> > +       long    tv_sec;         /* seconds */
> > +       long    tv_usec;        /* microseconds */
> > +};
> > +
> > +struct api_timespec {
> > +       long    tv_sec;         /* seconds */
> > +       long    tv_nsec;        /* microseconds */
> > +};
> > +
> > +typedef long api_time_t;
> > +
> >  static notrace cycle_t vread_hpet(void)
> >  {
> >         return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + HPET_COUNTER);
> >  }
> >
> > -notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
> > +notrace static long vdso_fallback_gettime(long clock, struct api_timespec *ts)
> >  {
> >         long ret;
> >         asm("syscall" : "=a" (ret) :
> > @@ -44,7 +56,8 @@ notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
> >         return ret;
> >  }
> >
> > -notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
> > +notrace static long vdso_fallback_gtod(struct api_timeval *tv,
> > +               struct timezone *tz)
> >  {
> >         long ret;
> >
> > @@ -54,20 +67,68 @@ notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
> >  }
> >  #else
> >
> > +#ifdef CONFIG_IA32_EMULATION
> > +typedef s64    arch_time_t;
> > +
> > +struct arch_timespec {
> > +       s64     tv_sec;
> > +       s64     tv_nsec;
> > +};
> > +
> > +#define ALIGN8 __attribute__ ((aligned (8)))
> > +
> > +struct arch_vsyscall_gtod_data {
> > +       seqcount_t      seq ALIGN8;
> > +
> > +       struct { /* extract of a clocksource struct */
> > +               int vclock_mode ALIGN8;
> > +               cycle_t cycle_last ALIGN8;
> > +               cycle_t mask ALIGN8;
> > +               u32     mult;
> > +               u32     shift;
> > +       } clock;
> > +
> > +       /* open coded 'struct timespec' */
> > +       arch_time_t     wall_time_sec;
> > +       u64             wall_time_snsec;
> > +       u64             monotonic_time_snsec;
> > +       arch_time_t     monotonic_time_sec;
> > +
> > +       struct timezone sys_tz;
> > +       struct arch_timespec wall_time_coarse;
> > +       struct arch_timespec monotonic_time_coarse;
> > +};
> 
> Yuck!
> 
> Can you see how hard it would be to just make the real gtod data have
> the same layout for 32-bit and 64-bit code?
> 

It is not easy, because the there are a lot of data types which use
longs (struct timespec, time_t) and seqcount has a variable size
depending on the kernel configuration.