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

[Jintack Lim]

On Fri, Feb 3, 2017 at 7:33 AM, Christoffer Dall <cdall@xxxxxxxxxx> wrote:
> On Thu, Feb 02, 2017 at 09:51:13AM -0500, Jintack Lim wrote:
>> On Thu, Feb 2, 2017 at 7:31 AM, Christoffer Dall <cdall@xxxxxxxxxx> wrote:
>> > 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.
>>
>> Ok. It's my fault not to specify that the emulated physical timer is
>> supported/tested on arm64.
>> On 32-bit platform, it is supposed to show the same behavior as
>> before, but I haven't tested.
>> Were you using the physical timer or the virtual timer for the guest?
>>
>> >
>> > Could you have a look?
>>
>> Sure, I'll have a look. I don't have access to my Cubietruck today,
>> but I can work on that tomorrow.
>>
>
> Don't bother, I've figured this out for you.

Thanks a lot.

>
> You need the following fixup to your patch:

Ok. I'll post v4 soon.
You've already do "acked-by" for this commit. Do I need to change it
to "signed-off-by"?

>
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 93c811c..35d7100 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -410,14 +410,21 @@ int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
>  }
>
>  /* Make the updates of cntvoff for all vtimer contexts atomic */
> -static void update_vtimer_cntvoff(struct kvm *kvm, u64 cntvoff)
> +static void update_vtimer_cntvoff(struct kvm_vcpu *vcpu, u64 cntvoff)
>  {
>         int i;
> -       struct kvm_vcpu *vcpu;
> +       struct kvm *kvm = vcpu->kvm;
> +       struct kvm_vcpu *tmp;
>
>         mutex_lock(&kvm->lock);
> -       kvm_for_each_vcpu(i, vcpu, kvm)
> -               vcpu_vtimer(vcpu)->cntvoff = cntvoff;
> +       kvm_for_each_vcpu(i, tmp, kvm)
> +               vcpu_vtimer(tmp)->cntvoff = cntvoff;
> +
> +       /*
> +        * When called from the vcpu create path, the CPU being created is not
> +        * included in the loop above, so we just set it here as well.
> +        */
> +       vcpu_vtimer(vcpu)->cntvoff = cntvoff;
>         mutex_unlock(&kvm->lock);
>  }
>
> @@ -426,7 +433,7 @@ void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
>         struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>
>         /* Synchronize cntvoff across all vtimers of a VM. */
> -       update_vtimer_cntvoff(vcpu->kvm, kvm_phys_timer_read());
> +       update_vtimer_cntvoff(vcpu, kvm_phys_timer_read());
>         vcpu_ptimer(vcpu)->cntvoff = 0;
>
>         INIT_WORK(&timer->expired, kvm_timer_inject_irq_work);
> @@ -448,7 +455,7 @@ int kvm_arm_timer_set_reg(struct kvm_vcpu *vcpu, u64 regid, u64 value)
>                 vtimer->cnt_ctl = value;
>                 break;
>         case KVM_REG_ARM_TIMER_CNT:
> -               update_vtimer_cntvoff(vcpu->kvm, kvm_phys_timer_read() - value);
> +               update_vtimer_cntvoff(vcpu, kvm_phys_timer_read() - value);
>                 break;
>         case KVM_REG_ARM_TIMER_CVAL:
>                 vtimer->cnt_cval = value;
>
> This is an amuzing one.

nice catch!

>
> Thanks,
> -Christoffer
>