Re: [RFC v2 02/10] KVM: arm/arm64: Move cntvoff to each timer context

[Marc Zyngier <marc.zyngier@arm.com>]

On 30/01/17 17:58, Jintack Lim wrote:
> On Sun, Jan 29, 2017 at 6:54 AM, Marc Zyngier <marc.zyngier@arm.com> wrote:
>> On Fri, Jan 27 2017 at 01:04:52 AM, Jintack Lim <jintack@cs.columbia.edu> wrote:
>>> Make cntvoff per each timer context. This is helpful to abstract kvm
>>> timer functions to work with timer context without considering timer
>>> types (e.g. physical timer or virtual timer).
>>>
>>> This also would pave the way for ever doing adjustments of the cntvoff
>>> on a per-CPU basis if that should ever make sense.
>>>
>>> Signed-off-by: Jintack Lim <jintack@cs.columbia.edu>
>>> ---
>>>  arch/arm/include/asm/kvm_host.h   |  6 +++---
>>>  arch/arm64/include/asm/kvm_host.h |  4 ++--
>>>  include/kvm/arm_arch_timer.h      |  8 +++-----
>>>  virt/kvm/arm/arch_timer.c         | 26 ++++++++++++++++++++------
>>>  virt/kvm/arm/hyp/timer-sr.c       |  3 +--
>>>  5 files changed, 29 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>>> index d5423ab..f5456a9 100644
>>> --- a/arch/arm/include/asm/kvm_host.h
>>> +++ b/arch/arm/include/asm/kvm_host.h
>>> @@ -60,9 +60,6 @@ struct kvm_arch {
>>>       /* The last vcpu id that ran on each physical CPU */
>>>       int __percpu *last_vcpu_ran;
>>>
>>> -     /* Timer */
>>> -     struct arch_timer_kvm   timer;
>>> -
>>>       /*
>>>        * Anything that is not used directly from assembly code goes
>>>        * here.
>>> @@ -75,6 +72,9 @@ struct kvm_arch {
>>>       /* Stage-2 page table */
>>>       pgd_t *pgd;
>>>
>>> +     /* A lock to synchronize cntvoff among all vtimer context of vcpus */
>>> +     spinlock_t cntvoff_lock;
>>
>> Is there any condition where we need this to be a spinlock? I would have
>> thought that a mutex should have been enough, as this should only be
>> updated on migration or initialization. Not that it matters much in this
>> case, but I wondered if there is something I'm missing.
>>
>>> +
>>>       /* Interrupt controller */
>>>       struct vgic_dist        vgic;
>>>       int max_vcpus;
>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>>> index e505038..23749a8 100644
>>> --- a/arch/arm64/include/asm/kvm_host.h
>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>> @@ -71,8 +71,8 @@ struct kvm_arch {
>>>       /* Interrupt controller */
>>>       struct vgic_dist        vgic;
>>>
>>> -     /* Timer */
>>> -     struct arch_timer_kvm   timer;
>>> +     /* A lock to synchronize cntvoff among all vtimer context of vcpus */
>>> +     spinlock_t cntvoff_lock;
>>>  };
>>>
>>>  #define KVM_NR_MEM_OBJS     40
>>> diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
>>> index daad3c1..1b9c988 100644
>>> --- a/include/kvm/arm_arch_timer.h
>>> +++ b/include/kvm/arm_arch_timer.h
>>> @@ -23,11 +23,6 @@
>>>  #include <linux/hrtimer.h>
>>>  #include <linux/workqueue.h>
>>>
>>> -struct arch_timer_kvm {
>>> -     /* Virtual offset */
>>> -     u64                     cntvoff;
>>> -};
>>> -
>>>  struct arch_timer_context {
>>>       /* Registers: control register, timer value */
>>>       u32                             cnt_ctl;
>>> @@ -38,6 +33,9 @@ struct arch_timer_context {
>>>
>>>       /* Active IRQ state caching */
>>>       bool                            active_cleared_last;
>>> +
>>> +     /* Virtual offset */
>>> +     u64                     cntvoff;
>>>  };
>>>
>>>  struct arch_timer_cpu {
>>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
>>> index 6740efa..fa4c042 100644
>>> --- a/virt/kvm/arm/arch_timer.c
>>> +++ b/virt/kvm/arm/arch_timer.c
>>> @@ -101,9 +101,10 @@ static void kvm_timer_inject_irq_work(struct work_struct *work)
>>>  static u64 kvm_timer_compute_delta(struct kvm_vcpu *vcpu)
>>>  {
>>>       u64 cval, now;
>>> +     struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>>>
>>> -     cval = vcpu_vtimer(vcpu)->cnt_cval;
>>> -     now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
>>> +     cval = vtimer->cnt_cval;
>>> +     now = kvm_phys_timer_read() - vtimer->cntvoff;
>>>
>>>       if (now < cval) {
>>>               u64 ns;
>>> @@ -159,7 +160,7 @@ bool kvm_timer_should_fire(struct kvm_vcpu *vcpu)
>>>               return false;
>>>
>>>       cval = vtimer->cnt_cval;
>>> -     now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
>>> +     now = kvm_phys_timer_read() - vtimer->cntvoff;
>>>
>>>       return cval <= now;
>>>  }
>>> @@ -353,10 +354,23 @@ int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
>>>       return 0;
>>>  }
>>>
>>> +/* Make the updates of cntvoff for all vtimer contexts atomic */
>>> +static void update_vtimer_cntvoff(struct kvm_vcpu *vcpu, u64 cntvoff)
>>
>> Arguably, this acts on the VM itself and not a single vcpu. maybe you
>> should consider passing the struct kvm pointer to reflect this.
>>
> 
> Yes, that would be better.
> 
>>> +{
>>> +     int i;
>>> +
>>> +     spin_lock(&vcpu->kvm->arch.cntvoff_lock);
>>> +     kvm_for_each_vcpu(i, vcpu, vcpu->kvm)
>>> +             vcpu_vtimer(vcpu)->cntvoff = cntvoff;
>>> +     spin_unlock(&vcpu->kvm->arch.cntvoff_lock);
>>> +}
>>> +
>>>  void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
>>>  {
>>>       struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>>>
>>> +     update_vtimer_cntvoff(vcpu, kvm_phys_timer_read());
>>
>> Maybe a comment indicating that we recompute CNTVOFF for all vcpus would
>> be welcome (this is not a change in semantics, but it was never obvious
>> in the existing code).
> 
> I'll add a comment. In fact, I was told to make cntvoff synchronized
> across all the vcpus, but I'm afraid that I understand why. Could you
> explain me where this constraint comes from?

The virtual counter is the only one a guest can rely on (as the physical
one is disabled). So we must present to the guest a view of time that is
uniform across CPUs. If we allow CNTVOFF to vary across CPUs, time
starts fluctuating when we migrate a process from a vcpu to another, and
Linux gets *really* unhappy.

An easy fix for this is to make CNTVOFF a VM-global value, ensuring that
all the CPUs see the same counter values at the same time.

Thanks,

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