Re: [PATCH v2 3/6] arm64/vdso: Add time napespace page

[Andrei Vagin]

On Tue, Apr 14, 2020 at 10:20 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Mon, Feb 24, 2020 at 11:37:28PM -0800, Andrei Vagin wrote:
> > Allocate the time namespace page among VVAR pages.  Provide
> > __arch_get_timens_vdso_data() helper for VDSO code to get the
> > code-relative position of VVARs on that special page.
> >
> > If a task belongs to a time namespace then the VVAR page which contains
> > the system wide VDSO data is replaced with a namespace specific page
> > which has the same layout as the VVAR page. That page has vdso_data->seq
> > set to 1 to enforce the slow path and vdso_data->clock_mode set to
> > VCLOCK_TIMENS to enforce the time namespace handling path.
> >
> > The extra check in the case that vdso_data->seq is odd, e.g. a concurrent
> > update of the VDSO data is in progress, is not really affecting regular
> > tasks which are not part of a time namespace as the task is spin waiting
> > for the update to finish and vdso_data->seq to become even again.
> >
> > If a time namespace task hits that code path, it invokes the corresponding
> > time getter function which retrieves the real VVAR page, reads host time
> > and then adds the offset for the requested clock which is stored in the
> > special VVAR page.
> >
> > Signed-off-by: Andrei Vagin <avagin@xxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/vdso.h                 |  6 ++++++
> >  .../include/asm/vdso/compat_gettimeofday.h    | 11 ++++++++++
> >  arch/arm64/include/asm/vdso/gettimeofday.h    |  8 ++++++++
> >  arch/arm64/kernel/vdso.c                      | 20 ++++++++++++++++---
> >  arch/arm64/kernel/vdso/vdso.lds.S             |  5 ++++-
> >  arch/arm64/kernel/vdso32/vdso.lds.S           |  5 ++++-
> >  include/vdso/datapage.h                       |  1 +
> >  7 files changed, 51 insertions(+), 5 deletions(-)
>
> > +#ifdef CONFIG_TIME_NS
> > +static __always_inline const struct vdso_data *__arch_get_timens_vdso_data(void)
> > +{
> > +     const struct vdso_data *ret;
> > +
> > +     asm volatile("mov %0, %1" : "=r"(ret) : "r"(_timens_data));
> > +
> > +     return ret;
> > +}
> > +#endif
>
> What is this inline assembly for? The commit message doesn't mention it,
> there's no explanation here, and the native version doesn't do likewise
> so it seems rather surprising.

__arch_get_vdso_data is  right before this function and there is a
comment which explains this:
https://github.com/torvalds/linux/blob/master/arch/arm64/include/asm/vdso/compat_gettimeofday.h#L137

"""
/*
* This simply puts &_vdso_data into ret. The reason why we don't use
* `ret = _vdso_data` is that the compiler tends to optimize this in a
* very suboptimal way: instead of keeping &_vdso_data in a register,
* it goes through a relocation almost every time _vdso_data must be
* accessed (even in subfunctions). This is both time and space
* consuming: each relocation uses a word in the code section, and it
* has to be loaded at runtime.
*
* This trick hides the assignment from the compiler. Since it cannot
* track where the pointer comes from, it will only use one relocation
* where __arch_get_vdso_data() is called, and then keep the result in
* a register.
*/
"""

I decided to not duplicate the comment because these two functions are
very similar and close to each other.

Thanks,
Andrei