[Patch] Use VDSO interface for gettimeofday on aarch64

[Szabolcs Nagy]

On 16/05/18 13:03, Szabolcs Nagy wrote:
> On 16/05/18 12:51, Adhemerval Zanella wrote:
>> On 16/05/2018 07:44, Szabolcs Nagy wrote:
>>> The other thing that would be nice to document is that
>>> why this change is safe for gettimeofday but not clock_gettime.
>>> (former does not have to set errno other than EFAULT but that
>>> case never works with vdso anyway, so the gettimeofday vdso
>>> function is a complete implementation, while clock_gettime
>>> has to deal with errno after the vdso call)
>>
>> As I put previously keep in mind that different that x86 and powerpc
>> implementations, where the vDSO symbol does not fail; the arm64 vDSO
>> implements a syscall fallback in case of underlying hardware requires
>> an out-of-line counter access (arch_timer_enable_workaround).
>>
>> Using a ifunc accessors to call vDSO directly will result in a slight
>> different semantic since generic implementation (kernel/time/time.c)
>> might return EFAULT in some cases (which won't be handled by ifunc
>> implementation).  This should not be an issue since POSIX [1] defines
>> no error code should reserved for the symbol, but it might trigger
>> some test in LTP.
>>
> 
> sorry i missed this comment,
> 
> if the vdso can fail with -EFAULT then this optimization is not valid
> (gettimeofday must either return 0 or -1).
> 
> if userspace can test whatever the kernel is testing there, that
> could be added to the ifunc resolver and only use vdso if we know
> it does not fail (i don't know if this is possible).
> 

hm in posix EFAULT is not a requirement, it's probably
undefined behaviour if gettimeofday is called with
wrong pointers. (this is borderline since linux man
pages document the EFAULT, but obviously that cannot
work for vdso anyway)

then the optimization is ok, but we definitely need
to document why we think directly going to the vdso
is safe.