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).