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

Christoffer Dall
2017-01-09T11:29:49Z

On Fri, Jan 06, 2017 at 03:16:24PM +0000, Marc Zyngier wrote:
> On 06/01/17 10:53, Christoffer Dall wrote:
> > On Fri, Jan 06, 2017 at 10:38:49AM +0000, Marc Zyngier wrote:
> >> On 06/01/17 10:00, Christoffer Dall wrote:
> >>> 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?
> >>
> >> Yes, they are. CNTVOFF_EL2 is ignored in that case, and you get an extra
> >> interrupt for the new EL2 virtual timer as well.
> >>
> >
> > ok, in that case I suppose I can just check for arch_timer_uses_ppi ==
> > VIRT_PPI and be done with it.
>
> I wonder what we should do for get_cycles(), which is hardwired to the
> virtual counter at the moment. If we decide to use the physical counter
> on systems identified as hosts, we may have to revise this as well (I
> feel the need for an alternative... ;-).
>

The physical counter is always available right? So why not just use the
physical counter?

I guess that comes down to two questions:

First, can get_cycles() be called from whereever in any context etc.?
In other words, is it ever called between vcpu_load() and vcpu_put(),
because if not, it doesn't matter which one we use, but that may be a
slightly brittle solution.

Second, what should the semantics of get_cycles be? Is it important
it's synchronized with the counter backing the timer being used, or
should it rather represents a wall-clock cycle measure, or should it
indeed represent virtual cycles spent if we ever decide to start
tweaking the offset depending on stolen cycles?

Perhaps I'm getting ahead of myself and we should just make sure it's
aligned with the timer in use, in which case we only have to decide if
we should implement that by (a) setting a function pointer, (b) using an
alternative, or (c) using a static key :)

Thanks,
-Christoffer