[PATCH 3/4] Add physical count arch timer support for clocksource in ARMv7.

[cinifr]

HI all,
      I have modified uboot code to switch monitor mode and to set
cntvoff for all smp cpus for allwinner a20 cpu. It works, kernel can
run ok without using cntpct. I will commit the path for uboot after
reviewing the code.
      Rong


On 13 September 2013 21:40, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 13/09/13 14:09, cinifr wrote:
>>> I urge you to read the ARM ARM, and specifically the section dedicated
>>> to trapping access to CP15 operations. If you do, you'll quickly notice
>>> that you *cannot* trap accesses to the timer subsystem.
>>>
>> I read it again. The ARMv7 manual said "Is accessible from Non-secure
>> PL1 modes only when CNTHCTL.PL1PCTEN is set to 1. When
>> CNTHCTL.PL1PCTEN is set to 0, any attempt to access CNTPCT from a
>> Non-secure PL1 mode ***generates a Hyp Trap exception***, see Hyp Trap
>> exception on page B1-1206" in B8.1.2. but I dont find  a  special hyp
>> trap control  for accessing CNTPCT in manual. As you said HSTR cannot
>> trap accessing of CP15 c14. What happer when OS access CNTPCT from PL1
>> NS=1 mode with CNTHCTL.PL1PCTEN=0 ???  AmI  wrong for understanding
>> the manual?
>
> That's interesting, as I never noticed this particular line in the ARM
> ARM. I'll investigate this, thanks for bringing it up.
>
> This doesn't change the fact that using the physical timer/counter in a
> VM is (or can be) horribly expensive, and should be avoided at all cost.
>
> Thanks,
>
>         M.
> --
> Jazz is not dead. It just smells funny...
>