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Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM

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Hi Jintack,

On Wed, Feb 01, 2017 at 12:43:00PM -0500, Jintack Lim wrote:
> The ARM architecture defines the EL1 physical timer and the virtual timer,
> and it is reasonable for an OS to expect to be able to access both.
> However, the current KVM implementation does not provide the EL1 physical
> timer to VMs but terminates VMs on access to the timer.
> 
> This patch series enables VMs to use the EL1 physical timer through
> trap-and-emulate.  The KVM host emulates each EL1 physical timer register
> access and sets up the background timer accordingly.  When the background
> timer expires, the KVM host injects EL1 physical timer interrupts to the
> VM.  Alternatively, it's also possible to allow VMs to access the EL1
> physical timer without trapping.  However, this requires somehow using the
> EL2 physical timer for the Linux host while running the VM instead of the
> EL1 physical timer.  Right now I just implemented trap-and-emulate because
> this was straightforward to do, and I leave it to future work to determine
> if transferring the EL1 physical timer state to the EL2 timer provides any
> performance benefit.
> 
> This feature will be useful for any OS that wishes to access the EL1
> physical timer. Nested virtualization is one of those use cases. A nested
> hypervisor running inside a VM would think it has full access to the
> hardware and naturally tries to use the EL1 physical timer as Linux would
> do. Other nested hypervisors may try to use the EL2 physical timer as Xen
> would do, but supporting the EL2 physical timer to the VM is out of scope
> of this patch series. This patch series will make it easy to add the EL2
> timer support in the future, though.
> 
> Note that Linux VMs booting in EL1 will be unaffected by this patch series
> and will continue to use only the virtual timer and this patch series will
> therefore not introduce any performance degredation as a result of
> trap-and-emulate.
> 
> v2 => v3:
>  - Rebase on kvmarm/queue
>  - Take kvm->lock to synchronize cntvoff across all vtimers
>  - Remove unnecessary function parameters
>  - Add comments

I just gave v3 a test run on my TC2 (32-bit platform) and my guest
quickly locks up trying to run cyclictest or when booting the machine it
stalls with RCU timeouts.

Could you have a look?

Thanks,
-Christoffer

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[RFC v3 00/10] Provide the EL1 physical timer to the VM Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
  [RFC v3 04/10] KVM: arm/arm64: Add the EL1 physical timer context Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
    Re: [RFC v3 04/10] KVM: arm/arm64: Add the EL1 physical timer context Christoffer Dall <cdall@linaro.org> - 2017-02-02 11:20 +0100
  [RFC v3 08/10] KVM: arm/arm64: Set up a background timer for the physical timer emulation Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
    Re: [RFC v3 08/10] KVM: arm/arm64: Set up a background timer for the  physical timer emulation Christoffer Dall <cdall@linaro.org> - 2017-02-02 11:40 +0100
  [RFC v3 07/10] KVM: arm/arm64: Set a background timer to the earliest timer expiration Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
    Re: [RFC v3 07/10] KVM: arm/arm64: Set a background timer to the  earliest timer expiration Christoffer Dall <cdall@linaro.org> - 2017-02-02 11:40 +0100
  [RFC v3 03/10] KVM: arm/arm64: Decouple kvm timer functions from virtual timer Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
    Re: [RFC v3 03/10] KVM: arm/arm64: Decouple kvm timer functions from  virtual timer Christoffer Dall <cdall@linaro.org> - 2017-02-02 11:10 +0100
  [RFC v3 02/10] KVM: arm/arm64: Move cntvoff to each timer context Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
    Re: [RFC v3 02/10] KVM: arm/arm64: Move cntvoff to each timer context Christoffer Dall <cdall@linaro.org> - 2017-02-02 11:10 +0100
  [RFC v3 01/10] KVM: arm/arm64: Abstract virtual timer context into separate structure Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
  [RFC v3 06/10] KVM: arm/arm64: Update the physical timer interrupt level Jintack Lim <jintack@cs.columbia.edu> - 2017-02-01 18:50 +0100
    Re: [RFC v3 06/10] KVM: arm/arm64: Update the physical timer  interrupt level Christoffer Dall <cdall@linaro.org> - 2017-02-02 11:20 +0100
  Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM Christoffer Dall <cdall@linaro.org> - 2017-02-02 13:40 +0100
    Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM Jintack Lim <jintack@cs.columbia.edu> - 2017-02-02 16:00 +0100
      Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM Christoffer Dall <cdall@linaro.org> - 2017-02-02 16:10 +0100
      Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM Christoffer Dall <cdall@linaro.org> - 2017-02-03 13:40 +0100
        Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM Jintack Lim <jintack@cs.columbia.edu> - 2017-02-03 14:20 +0100
          Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM Christoffer Dall <cdall@linaro.org> - 2017-02-03 14:40 +0100

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