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

Catalin Marinas <catalin.marinas <at> arm.com> writes:

On Fri, Feb 13, 2015 at 04:36:25AM +0000, Venkappa Mala wrote:

quoted

Nathan Lynch <nathan_lynch <at> mentor.com> writes:

quoted

+#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, 

quoted

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.

[...]

quoted

Perhaps, either we need to revise the vDSO frame work to choose VCT 

or 

quoted

PCT dynamically Otherwise CNTVOFF reset to be zero but CNTVOFF could 

not 

quoted

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?