Subject: [PATCH v10 4/6] ARM: add vdso user-space code

From: [email-masked] (Catalin Marinas)
Date: Fri, 13 Feb 2015 14:14:26 +0000

On Fri, Feb 13, 2015 at 12:01:40PM +0000, Venkappa Mala wrote:
> Catalin Marinas <catalin.marinas <at> arm.com> writes:
> > On Fri, Feb 13, 2015 at 04:36:25AM +0000, Venkappa Mala wrote:
> > > Nathan Lynch <nathan_lynch <at> mentor.com> writes:
> > > > +#ifdef CONFIG_ARM_ARCH_TIMER
> > > > +
> > > > +static notrace u64 get_ns(struct vdso_data *vdata)
> > > > +{
> > > > +	u64 cycle_delta;
> > > > +	u64 cycle_now;
> > > > +	u64 nsec;
> > > > +
> > > > +	cycle_now = arch_counter_get_cntvct();
> > > 
> > > Regarding ARM:vDSO, I have enabled your patch set on Cortex-A7/ARMv7, 
> > > the vDSO is up and running using generic arch timer using physical 
> > > counter but not with VCT.
> > > I anticipate, the issue could be due to VCT cycles.
> > [...]
> > > Perhaps, either we need to revise the vDSO frame work to choose VCT or 
> > > PCT dynamically Otherwise CNTVOFF reset to be zero but CNTVOFF could not 
> > > be accessed in SVC mode (PL1).
> > 
> > I assume on your platform, the DT also contains
> > "arm,cpu-registers-not-fw-configured". In such case, the VDSO
> > optimisation for gettimeofday should just be disabled (as it would be if
> > the generic timers are not present). Really, just fix the firmware.
> 
> Thanks Catalin for your promote reply. Yes, DTS contained
> arm,cpu-registers-not-fw-configured so that it uses generic physical
> timer instead of virtual timer. Now, the vDSO gettimeofday
> optimization is also possible since the generic timer is presented and
> enabled. However, vDSO gettimeofday implementation has been using
> virtual timer (VCT  user access) but the current implementation will
> not be sufficient to handle when the firmware is not taken care about
> reset CNTVOFF to zero. Let?s assume,neither firmware taken care nor
> booted with HYP mode, then Can we use physical timer (PCT user access
> needs to be enabled) for vDSO in the safe manner?

The short answer: no.

The problem is that user space is very "innovative" in making use of
features in a different way than what the kernel people intended and
it will be claimed "user ABI" afterwards. Take this glibc revert for
example, luckily we noticed it early:

https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=33ef2f0c763b51e1df7896d7d39d585824558c75

CNTVCT is not intended to be used outside of the VDSO but, apparently,
the VDSO->kernel interface is seen as user ABI by some.

CNTPCT cannot always be used in user space (VDSO and not even the
kernel), especially when the kernel boots at EL1. Choosing CNTPCT vs
CNTVCT in VDSO dynamically (or at boot time) is technically doable but
there is a high risk that user space ends up using them directly when it
detects a gettimeofday VDSO and such code will break when CNTPCT is no
longer accessible.

-- 
Catalin