Re: [PATCH 2 of 5] Improve ring management for memory events. Do not lose guest events

[Olaf Hering <olaf@aepfle.de>]

On Tue, Nov 29, Andres Lagar-Cavilla wrote:

> @@ -133,31 +185,95 @@ static int mem_event_disable(struct mem_
>      return 0;
>  }
>  
> -void mem_event_put_request(struct domain *d, struct mem_event_domain *med, mem_event_request_t *req)
> +static inline int mem_event_ring_free(struct domain *d, struct mem_event_domain *med)
>  {
> +    int free_requests;
> +
> +    free_requests = RING_FREE_REQUESTS(&med->front_ring);
> +    if ( unlikely(free_requests < d->max_vcpus) )
> +    {
> +        /* This may happen during normal operation (hopefully not often). */
> +        gdprintk(XENLOG_INFO, "mem_event request slots for domain %d: %d\n",
> +                               d->domain_id, free_requests);
> +    }
> +
> +    return free_requests;
> +}
> +
> +/* Return values
> + * zero: success
> + * -ENOSYS: no ring
> + * -EAGAIN: ring is full and the event has not been transmitted.
> + *          Only foreign vcpu's get EAGAIN
> + * -EBUSY: guest vcpu has been paused due to ring congestion
> + */ 
> +int mem_event_put_request(struct domain *d, struct mem_event_domain *med, mem_event_request_t *req)
> +{
> +    int ret = 0;
> +    int foreign = (d != current->domain);

> +    /*
> +     * We ensure that each vcpu can put at least *one* event -- because some
> +     * events are not repeatable, such as dropping a page.  This will ensure no
> +     * vCPU is left with an event that they must place on the ring, but cannot.
> +     * They will be paused after the event is placed.
> +     * See large comment below in mem_event_unpause_vcpus().
> +     */
> +    if ( !foreign && mem_event_ring_free(d, med) < d->max_vcpus )
> +    {
> +        mem_event_mark_and_pause(current, med);
> +        ret = -EBUSY;
> +    }
>  
>      mem_event_ring_unlock(med);
>  
>      notify_via_xen_event_channel(d, med->xen_port);
> +
> +    return ret;


What will happen if the guest has more vcpus than r->nr_ents in the ring
buffer? To me it looks like no event can be placed into the ring and
-EBUSY is returned instead.

Olaf