Re: [PATCH v3 05/11] x86/vioapic: switch to use the EOI callback mechanism

[Jan Beulich <jbeulich@suse.com>]

On 31.03.2021 12:32, Roger Pau Monne wrote:
> --- a/xen/arch/x86/hvm/vioapic.c
> +++ b/xen/arch/x86/hvm/vioapic.c
> @@ -394,6 +394,50 @@ static const struct hvm_mmio_ops vioapic_mmio_ops = {
>      .write = vioapic_write
>  };
>  
> +static void eoi_callback(unsigned int vector, void *data)
> +{
> +    struct domain *d = current->domain;
> +    struct hvm_irq *hvm_irq = hvm_domain_irq(d);
> +    unsigned int i;
> +
> +    ASSERT(has_vioapic(d));

On the same grounds on which you dropped checks from hvm_dpci_msi_eoi()
in the previous patch you could imo now drop this assertion.

> @@ -621,7 +624,43 @@ static int ioapic_load(struct domain *d, hvm_domain_context_t *h)
>           d->arch.hvm.nr_vioapics != 1 )
>          return -EOPNOTSUPP;
>  
> -    return hvm_load_entry(IOAPIC, h, &s->domU);
> +    rc = hvm_load_entry(IOAPIC, h, &s->domU);
> +    if ( rc )
> +        return rc;
> +
> +    for ( i = 0; i < ARRAY_SIZE(s->domU.redirtbl); i++ )
> +    {
> +        const union vioapic_redir_entry *ent = &s->domU.redirtbl[i];
> +        unsigned int vector = ent->fields.vector;
> +        unsigned int delivery_mode = ent->fields.delivery_mode;
> +        struct vcpu *v;
> +
> +        /*
> +         * Add a callback for each possible vector injected by a redirection
> +         * entry.
> +         */
> +        if ( vector < 16 || !ent->fields.remote_irr ||
> +             (delivery_mode != dest_LowestPrio && delivery_mode != dest_Fixed) )
> +            continue;
> +
> +        for_each_vcpu ( d, v )
> +        {
> +            struct vlapic *vlapic = vcpu_vlapic(v);
> +
> +            /*
> +             * NB: if the vlapic registers were restored before the vio-apic
> +             * ones we could test whether the vector is set in the vlapic IRR
> +             * or ISR registers before unconditionally setting the callback.
> +             * This is harmless as eoi_callback is capable of dealing with
> +             * spurious callbacks.
> +             */
> +            if ( vlapic_match_dest(vlapic, NULL, 0, ent->fields.dest_id,
> +                                   ent->fields.dest_mode) )
> +                vlapic_set_callback(vlapic, vector, eoi_callback, NULL);

eoi_callback()'s behavior is only one of the aspects to consider here.
The other is vlapic_set_callback()'s complaining if it finds a
callback already set. What guarantees that a mistakenly set callback
here won't get in conflict with some future use of the same vector by
the guest?

And btw - like in the earlier patch you could again pass d instead of
NULL here, avoiding the need to establish it from current in the
callback.

> --- a/xen/arch/x86/hvm/vlapic.c
> +++ b/xen/arch/x86/hvm/vlapic.c
> @@ -192,7 +192,13 @@ void vlapic_set_irq_callback(struct vlapic *vlapic, uint8_t vec, uint8_t trig,
>  
>      if ( hvm_funcs.update_eoi_exit_bitmap )
>          alternative_vcall(hvm_funcs.update_eoi_exit_bitmap, target, vec,
> -                          trig || callback);
> +                          /*
> +                           * NB: need to explicitly convert to boolean to avoid
> +                           * truncation wrongly result in false begin reported
> +                           * for example when the pointer sits on a page
> +                           * boundary.
> +                           */
> +                          !!callback);

I've had quite a bit of difficulty with the comment. Once I realized
that you likely mean "being" instead of "begin" it got a bit better.
I'd like to suggest also s/result/resulting/, a comma after "reported",
and maybe then s/being reported/getting passed/.

As to explicitly converting to bool, wouldn't a cast to bool do? That's
more explicitly an "explicit conversion" than using !!.

Jan