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

[Marc Zyngier]

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 at 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.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...