Re: [RFC PATCH 1/7] arm64: Use physical counter for in-kernel reads

Christoffer Dall
2017-01-06T10:00:49Z

Hi Marc,

On Thu, Jan 05, 2017 at 06:11:14PM +0000, Marc Zyngier wrote:
> [adding the arm64 maintainers, plus Mark as arch timer maintainer]

Right, sorry, I should have done that already.

>
> On 10/12/16 20:47, Christoffer Dall wrote:
> > Using the physical counter allows KVM to retain the offset between the
> > virtual and physical counter as long as it is actively running a VCPU.
> >
> > As soon as a VCPU is released, another thread is scheduled or we start
> > running userspace applications, we reset the offset to 0, so that VDSO
> > operations can still read the virtual counter and get the same view of
> > time as the kernel.
> >
> > This opens up potential improvements for KVM performance.
> >
> > VHE kernels or kernels using the virtual timer are unaffected by this.
> >
> > Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> > ---
> > arch/arm64/include/asm/arch_timer.h | 6 ++++--
> > drivers/clocksource/arm_arch_timer.c | 2 +-
> > 2 files changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> > index eaa5bbe..cec2549 100644
> > --- a/arch/arm64/include/asm/arch_timer.h
> > +++ b/arch/arm64/include/asm/arch_timer.h
> > @@ -139,11 +139,13 @@ static inline void arch_timer_set_cntkctl(u32 cntkctl)
> >
> > static inline u64 arch_counter_get_cntpct(void)
> > {
> > + u64 cval;
> > /*
> > * AArch64 kernel and user space mandate the use of CNTVCT.
> > */
> > - BUG();
> > - return 0;
> > + isb();
> > + asm volatile("mrs %0, cntpct_el0" : "=r" (cval));
> > + return cval;
> > }
> >
> > static inline u64 arch_counter_get_cntvct(void)
> > diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> > index 73c487d..a5b0789 100644
> > --- a/drivers/clocksource/arm_arch_timer.c
> > +++ b/drivers/clocksource/arm_arch_timer.c
> > @@ -597,7 +597,7 @@ static void __init arch_counter_register(unsigned type)
> >
> > /* Register the CP15 based counter if we have one */
> > if (type & ARCH_CP15_TIMER) {
> > - if (IS_ENABLED(CONFIG_ARM64) || arch_timer_uses_ppi == VIRT_PPI)
> > + if (arch_timer_uses_ppi == VIRT_PPI || is_kernel_in_hyp_mode())
>
> Why do we have this is_kernel_in_hyp_mode clause? I can't think of any
> reason for a VHE kernel to use the virtual counter at all...
>

Good question. I think I just didn't want to change behavior from the
existing functionality mre than necessary.

Note that on a VHE kernel this will be the EL2 virtual counter, not the
EL1 virtual counter, due to the register redirection. Are the virtual
and physical EL2 counters always equivalent on a VHE system?

> > arch_timer_read_counter = arch_counter_get_cntvct;
> > else
> > arch_timer_read_counter = arch_counter_get_cntpct;
> >
>

Thanks,
-Christoffer