Re: [Xen-devel] [PATCH v2 7/8] xen: arm: context switch vtimer PPI state.

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 10 Nov 2015, Ian Campbell wrote:
> ... instead of artificially masking the timer interrupt in the timer
> state and relying on the guest to unmask (which it isn't required to
> do per the h/w spec, although Linux does).
> 
> By using the newly added hwppi save/restore functionality we make use
> of the GICD I[SC]ACTIVER registers to save and restore the active
> state of the interrupt, which prevents the nested invocations which
> the current masking works around.
> 
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> ---
> v2: Rebased, in particular over Julien's passthrough stuff which
>     reworked a bunch of IRQ related stuff.
>     Also largely rewritten since precursor patches now lay very
>     different groundwork.
> ---
>  xen/arch/arm/time.c          | 26 ++------------------------
>  xen/arch/arm/vtimer.c        | 41 +++++++++++++++++++++++++++++++++++++----
>  xen/include/asm-arm/domain.h |  1 +
>  3 files changed, 40 insertions(+), 28 deletions(-)
> 
> diff --git a/xen/arch/arm/time.c b/xen/arch/arm/time.c
> index 5ded30c..2a1cdba 100644
> --- a/xen/arch/arm/time.c
> +++ b/xen/arch/arm/time.c
> @@ -181,28 +181,6 @@ static void timer_interrupt(int irq, void *dev_id, 
> struct cpu_user_regs *regs)
>      }
>  }
>  
> -static void vtimer_interrupt(int irq, void *dev_id, struct cpu_user_regs 
> *regs)
> -{
> -    /*
> -     * Edge-triggered interrupts can be used for the virtual timer. Even
> -     * if the timer output signal is masked in the context switch, the
> -     * GIC will keep track that of any interrupts raised while IRQS are
> -     * disabled. As soon as IRQs are re-enabled, the virtual interrupt
> -     * will be injected to Xen.
> -     *
> -     * If an IDLE vCPU was scheduled next then we should ignore the
> -     * interrupt.
> -     */
> -    if ( unlikely(is_idle_vcpu(current)) )
> -        return;
> -
> -    perfc_incr(virt_timer_irqs);
> -
> -    current->arch.virt_timer.ctl = READ_SYSREG32(CNTV_CTL_EL0);
> -    WRITE_SYSREG32(current->arch.virt_timer.ctl | CNTx_CTL_MASK, 
> CNTV_CTL_EL0);
> -    vgic_vcpu_inject_irq(current, current->arch.virt_timer.irq);
> -}
> -
>  /*
>   * Arch timer interrupt really ought to be level triggered, since the
>   * design of the timer/comparator mechanism is based around that
> @@ -242,8 +220,8 @@ void __cpuinit init_timer_interrupt(void)
>  
>      request_irq(timer_irq[TIMER_HYP_PPI], 0, timer_interrupt,
>                  "hyptimer", NULL);
> -    request_irq(timer_irq[TIMER_VIRT_PPI], 0, vtimer_interrupt,
> -                   "virtimer", NULL);
> +    route_hwppi_to_current_vcpu(timer_irq[TIMER_VIRT_PPI], "virtimer");
> +
>      request_irq(timer_irq[TIMER_PHYS_NONSECURE_PPI], 0, timer_interrupt,
>                  "phytimer", NULL);
>  
> diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
> index 1418092..82e2b88 100644
> --- a/xen/arch/arm/vtimer.c
> +++ b/xen/arch/arm/vtimer.c
> @@ -55,9 +55,19 @@ static void phys_timer_expired(void *data)
>  static void virt_timer_expired(void *data)
>  {
>      struct vtimer *t = data;
> -    t->ctl |= CNTx_CTL_MASK;
> -    vgic_vcpu_inject_irq(t->v, t->irq);
> -    perfc_incr(vtimer_virt_inject);
> +    t->ctl |= CNTx_CTL_PENDING;
> +    if ( !(t->ctl & CNTx_CTL_MASK) )
> +    {
> +        /*
> +         * An edge triggered interrupt should now be pending. Since
> +         * this timer can never expire while the domain is scheduled
> +         * we know that the gic_restore_hwppi in virt_timer_restore
> +         * will cause the real hwppi to occur and be routed.
> +         */
> +        gic_hwppi_set_pending(&t->ppi_state);

I wonder if calling gic_hwppi_set_pending is actually redundant:
virt_timer_restore will write the expired time to cval, causing a new
interrupt to be immediately sent as soon as it's enabled, right?


> +        vcpu_unblock(t->v);
> +        perfc_incr(vtimer_virt_inject);
> +    }
>  }
>  
>  int domain_vtimer_init(struct domain *d, struct xen_arch_domainconfig 
> *config)
> @@ -96,9 +106,12 @@ int domain_vtimer_init(struct domain *d, struct 
> xen_arch_domainconfig *config)
>  
>  int vcpu_vtimer_init(struct vcpu *v)
>  {
> +    struct pending_irq *p;
>      struct vtimer *t = &v->arch.phys_timer;
>      bool_t d0 = is_hardware_domain(v->domain);
>  
> +    const unsigned host_vtimer_irq_ppi = timer_get_irq(TIMER_VIRT_PPI);
> +
>      /*
>       * Hardware domain uses the hardware interrupts, guests get the virtual
>       * platform.
> @@ -116,10 +129,16 @@ int vcpu_vtimer_init(struct vcpu *v)
>      init_timer(&t->timer, virt_timer_expired, t, v->processor);
>      t->ctl = 0;
>      t->irq = d0
> -        ? timer_get_irq(TIMER_VIRT_PPI)
> +        ? host_vtimer_irq_ppi
>          : GUEST_TIMER_VIRT_PPI;
>      t->v = v;
>  
> +    p = irq_to_pending(v, t->irq);
> +    p->irq = t->irq;
> +
> +    gic_hwppi_state_init(&v->arch.virt_timer.ppi_state,
> +                         host_vtimer_irq_ppi);
> +
>      v->arch.vtimer_initialized = 1;
>  
>      return 0;
> @@ -147,6 +166,15 @@ int virt_timer_save(struct vcpu *v)
>          set_timer(&v->arch.virt_timer.timer, 
> ticks_to_ns(v->arch.virt_timer.cval +
>                    v->domain->arch.virt_timer_base.offset - boot_count));
>      }
> +
> +    /*
> +     * Since the vtimer irq is a PPI we don't need to worry about
> +     * racing against it becoming active while we are saving the
> +     * state, since that requires the guest to be reading the IAR.
> +     */
> +    gic_save_and_mask_hwppi(v, v->arch.virt_timer.irq,
> +                            &v->arch.virt_timer.ppi_state);
> +
>      return 0;
>  }
>  
> @@ -161,6 +189,11 @@ int virt_timer_restore(struct vcpu *v)
>      WRITE_SYSREG64(v->domain->arch.virt_timer_base.offset, CNTVOFF_EL2);
>      WRITE_SYSREG64(v->arch.virt_timer.cval, CNTV_CVAL_EL0);
>      WRITE_SYSREG32(v->arch.virt_timer.ctl, CNTV_CTL_EL0);
> +
> +    gic_restore_hwppi(v,
> +                      v->arch.virt_timer.irq,
> +                      &v->arch.virt_timer.ppi_state);
> +
>      return 0;
>  }
>  
> diff --git a/xen/include/asm-arm/domain.h b/xen/include/asm-arm/domain.h
> index 550e28b..aff21dd 100644
> --- a/xen/include/asm-arm/domain.h
> +++ b/xen/include/asm-arm/domain.h
> @@ -51,6 +51,7 @@ struct vtimer {
>      struct timer timer;
>      uint32_t ctl;
>      uint64_t cval;
> +    struct hwppi_state ppi_state;
>  };
>  
>  struct arch_domain
> -- 
> 2.1.4
> 

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