Re: [RFC 8/8] KVM: arm/arm64: Emulate the EL1 phys timer register access

[Jintack Lim]

On Mon, Jan 9, 2017 at 7:16 AM, Christoffer Dall
<christoffer.dall@xxxxxxxxxx> wrote:
> On Mon, Dec 26, 2016 at 12:12:06PM -0500, Jintack Lim wrote:
>> Emulate read and write operations to CNTP_TVAL, CNTP_CVAL and CNTP_CTL.
>> Now the VM is able to use the EL1 physical timer.
>>
>> Signed-off-by: Jintack Lim <jintack@xxxxxxxxxxxxxxx>
>> ---
>>  arch/arm64/kvm/sys_regs.c    | 35 ++++++++++++++++++++++++++++++++---
>>  include/kvm/arm_arch_timer.h |  3 +++
>>  virt/kvm/arm/arch_timer.c    |  4 ++--
>>  3 files changed, 37 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index fd9e747..7cef94f 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -824,7 +824,15 @@ static bool access_cntp_tval(struct kvm_vcpu *vcpu,
>>               struct sys_reg_params *p,
>>               const struct sys_reg_desc *r)
>>  {
>> -     kvm_inject_undefined(vcpu);
>> +     struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
>> +     cycle_t now = kvm_phys_timer_read();
>> +
>> +     if (p->is_write) {
>> +             ptimer->cnt_cval = p->regval + now;
>> +             kvm_timer_emulate(vcpu, ptimer);
>
> Hmm, do we really need those calls here?
>
> I guess I'm a little confused about exactly what the kvm_timer_emulate()
> function is supposed to do, and it feels to me like these handlers
> should just record what the guest is asking the kernel to do and the
> logic of handling the additional timer should be moved into the run path
> as much as possible.

I think it's a design decision. As you suggested, it's simple to do
set up the background timer on entry to the VM, cancel it on exit, but
since that's on the critical path it may have some impact on the
performance, especially the world switch cost. To avoid
canceling/setting up timer every world switch, I choose to schedule
the physical timer here. I haven't compared the cost of the two
alternatives, though.

>
> Thanks,
> -Christoffer
>
>> +     } else
>> +             p->regval = ptimer->cnt_cval - now;
>> +
>>       return true;
>>  }
>>
>> @@ -832,7 +840,21 @@ static bool access_cntp_ctl(struct kvm_vcpu *vcpu,
>>               struct sys_reg_params *p,
>>               const struct sys_reg_desc *r)
>>  {
>> -     kvm_inject_undefined(vcpu);
>> +     struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
>> +
>> +     if (p->is_write) {
>> +             /* ISTATUS bit is read-only */
>> +             ptimer->cnt_ctl = p->regval & ~ARCH_TIMER_CTRL_IT_STAT;
>> +             kvm_timer_emulate(vcpu, ptimer);
>> +     } else {
>> +             cycle_t now = kvm_phys_timer_read();
>> +
>> +             p->regval = ptimer->cnt_ctl;
>> +             /* Set ISTATUS bit if it's expired */
>> +             if (ptimer->cnt_cval <= now)
>> +                     p->regval |= ARCH_TIMER_CTRL_IT_STAT;
>> +     }
>> +
>>       return true;
>>  }
>>
>> @@ -840,7 +862,14 @@ static bool access_cntp_cval(struct kvm_vcpu *vcpu,
>>               struct sys_reg_params *p,
>>               const struct sys_reg_desc *r)
>>  {
>> -     kvm_inject_undefined(vcpu);
>> +     struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
>> +
>> +     if (p->is_write) {
>> +             ptimer->cnt_cval = p->regval;
>> +             kvm_timer_emulate(vcpu, ptimer);
>> +     } else
>> +             p->regval = ptimer->cnt_cval;
>> +
>>       return true;
>>  }
>>
>> diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
>> index 04ed9c1..776579b 100644
>> --- a/include/kvm/arm_arch_timer.h
>> +++ b/include/kvm/arm_arch_timer.h
>> @@ -75,6 +75,9 @@ bool kvm_timer_should_fire(struct kvm_vcpu *vcpu,
>>                          struct arch_timer_context *timer_ctx);
>>  void kvm_timer_schedule(struct kvm_vcpu *vcpu);
>>  void kvm_timer_unschedule(struct kvm_vcpu *vcpu);
>> +void kvm_timer_emulate(struct kvm_vcpu *vcpu, struct arch_timer_context *timer);
>> +
>> +cycle_t kvm_phys_timer_read(void);
>>
>>  void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu);
>>
>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
>> index be8d953..7a161f8 100644
>> --- a/virt/kvm/arm/arch_timer.c
>> +++ b/virt/kvm/arm/arch_timer.c
>> @@ -39,7 +39,7 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
>>       vcpu_vtimer(vcpu)->active_cleared_last = false;
>>  }
>>
>> -static cycle_t kvm_phys_timer_read(void)
>> +cycle_t kvm_phys_timer_read(void)
>>  {
>>       return timecounter->cc->read(timecounter->cc);
>>  }
>> @@ -258,7 +258,7 @@ static int kvm_timer_update_state(struct kvm_vcpu *vcpu)
>>   * Schedule the background timer for the emulated timer. The background timer
>>   * runs whenever vcpu is runnable and the timer is not expired.
>>   */
>> -static void kvm_timer_emulate(struct kvm_vcpu *vcpu,
>> +void kvm_timer_emulate(struct kvm_vcpu *vcpu,
>>                      struct arch_timer_context *timer_ctx)
>>  {
>>       struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>> --
>> 1.9.1
>>
>>
>