C vdso

[Christophe Leroy]

Hi Michael

Le 17/09/2020 à 14:33, Michael Ellerman a écrit :
> Hi Christophe,
> 
> Christophe Leroy <christophe.leroy at csgroup.eu> writes:
>> Hi Michael,
>>
>> What is the status with the generic C vdso merge ?
>> In some mail, you mentionned having difficulties getting it working on
>> ppc64, any progress ? What's the problem ? Can I help ?
> 
> Yeah sorry I was hoping to get time to work on it but haven't been able
> to.
> 
> It's causing crashes on ppc64 ie. big endian.
> 

> 
> As you can see from the instruction dump we have jumped into the weeds somewhere.
> 
> We also had the report from the kbuild robot about rela.opd being
> discarded, which I think is indicative of a bigger problem. ie. we don't
> process relocations for the VDSO, but opds require relocations (they
> contain an absolute pointer).
> 
> I thought we could get away with that, because the VDSO entry points
> aren't proper functions (so they don't have opds), and I didn't think
> we'd be calling via function pointers in the VDSO code (which would
> require opds). But seems something is not working right.
> 
> Sorry I haven't got back to you with those details. Things are a bit of
> a mess inside IBM at the moment (always?), and I've been trying to get
> everything done before I take a holiday next week.
> 


Can you tell what defconfig you are using ? I have been able to setup a 
full glibc PPC64 cross compilation chain and been able to test it under 
QEMU with success, using Nathan's vdsotest tool.

I tested with both ppc64_defconfig and pseries_defconfig.

The only problem I got is with getcpu, which segfaults but both before 
and after applying my series, so I guess this is unrelated.

Not sure we can pay too much attention to the exact measurement as it is 
a ppc64 QEMU running on a x86 Linux which is running in a Virtual Box on 
a x86 windows Laptop, but at least it works:

Without the series:

clock-getres-monotonic:    vdso: 389 nsec/call
clock-gettime-monotonic:    vdso: 781 nsec/call
clock-getres-monotonic-coarse:    vdso: 13715 nsec/call
clock-gettime-monotonic-coarse:    vdso: 312 nsec/call
clock-getres-monotonic-raw:    vdso: 13589 nsec/call
clock-getres-tai:    vdso: 13827 nsec/call
clock-gettime-tai:    vdso: 14846 nsec/call
clock-getres-boottime:    vdso: 13596 nsec/call
clock-gettime-boottime:    vdso: 14758 nsec/call
clock-getres-realtime:    vdso: 327 nsec/call
clock-gettime-realtime:    vdso: 717 nsec/call
clock-getres-realtime-coarse:    vdso: 14102 nsec/call
clock-gettime-realtime-coarse:    vdso: 299 nsec/call
gettimeofday:    vdso: 771 nsec/call

With the series:

clock-getres-monotonic:    vdso: 350 nsec/call
clock-gettime-monotonic:    vdso: 726 nsec/call
clock-getres-monotonic-coarse:    vdso: 356 nsec/call
clock-gettime-monotonic-coarse:    vdso: 423 nsec/call
clock-getres-monotonic-raw:    vdso: 349 nsec/call
clock-getres-tai:    vdso: 419 nsec/call
clock-gettime-tai:    vdso: 724 nsec/call
clock-getres-boottime:    vdso: 352 nsec/call
clock-gettime-boottime:    vdso: 752 nsec/call
clock-getres-realtime:    vdso: 351 nsec/call
clock-gettime-realtime:    vdso: 733 nsec/call
clock-getres-realtime-coarse:    vdso: 356 nsec/call
clock-gettime-realtime-coarse:    vdso: 367 nsec/call
gettimeofday:    vdso: 796 nsec/call


Thanks
Christophe