On Thu, Feb 9, 2017 at 10:11 AM, Razvan Cojocaru
<rcojoc...@bitdefender.com> wrote:
> It is currently possible for the guest to lock when subscribing
> to synchronous vm_events if max_vcpus is larger than the
> number of available ring buffer slots. This patch no longer
> blocks already paused VCPUs, fixing the issue for this use
> case, and wakes up as many blocked VCPUs as there are slots
> available in the ring buffer, eliminating the blockage for
> asynchronous events.
>
> Signed-off-by: Razvan Cojocaru <rcojoc...@bitdefender.com>
Acked-by: Tamas K Lengyel <ta...@tklengyel.com>
>
> ---
> Changes since V2:
> - Fixed typo: re-enabled commented-out code (for testing).
> ---
> xen/common/vm_event.c | 27 +++++++--------------------
> 1 file changed, 7 insertions(+), 20 deletions(-)
>
> diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
> index 82ce8f1..45046d1 100644
> --- a/xen/common/vm_event.c
> +++ b/xen/common/vm_event.c
> @@ -127,26 +127,11 @@ static unsigned int vm_event_ring_available(struct
> vm_event_domain *ved)
> static void vm_event_wake_blocked(struct domain *d, struct vm_event_domain
> *ved)
> {
> struct vcpu *v;
> - int online = d->max_vcpus;
> unsigned int avail_req = vm_event_ring_available(ved);
>
> if ( avail_req == 0 || ved->blocked == 0 )
> return;
>
> - /*
> - * We ensure that we only have vCPUs online if there are enough free
> slots
> - * for their memory events to be processed. This will ensure that no
> - * memory events are lost (due to the fact that certain types of events
> - * cannot be replayed, we need to ensure that there is space in the ring
> - * for when they are hit).
> - * See comment below in vm_event_put_request().
> - */
> - for_each_vcpu ( d, v )
> - if ( test_bit(ved->pause_flag, &v->pause_flags) )
> - online--;
> -
> - ASSERT(online == (d->max_vcpus - ved->blocked));
> -
> /* We remember which vcpu last woke up to avoid scanning always linearly
> * from zero and starving higher-numbered vcpus under high load */
> if ( d->vcpu )
> @@ -160,13 +145,13 @@ static void vm_event_wake_blocked(struct domain *d,
> struct vm_event_domain *ved)
> if ( !v )
> continue;
>
> - if ( !(ved->blocked) || online >= avail_req )
> + if ( !(ved->blocked) || avail_req == 0 )
> break;
>
> if ( test_and_clear_bit(ved->pause_flag, &v->pause_flags) )
> {
> vcpu_unpause(v);
> - online++;
> + avail_req--;
> ved->blocked--;
> ved->last_vcpu_wake_up = k;
> }
> @@ -280,8 +265,9 @@ void vm_event_put_request(struct domain *d,
> int free_req;
> unsigned int avail_req;
> RING_IDX req_prod;
> + struct vcpu *curr = current;
>
> - if ( current->domain != d )
> + if ( curr->domain != d )
> {
> req->flags |= VM_EVENT_FLAG_FOREIGN;
> #ifndef NDEBUG
> @@ -316,8 +302,9 @@ void vm_event_put_request(struct domain *d,
> * See the comments above wake_blocked() for more information
> * on how this mechanism works to avoid waiting. */
> avail_req = vm_event_ring_available(ved);
> - if( current->domain == d && avail_req < d->max_vcpus )
> - vm_event_mark_and_pause(current, ved);
> + if( curr->domain == d && avail_req < d->max_vcpus &&
> + !atomic_read(&curr->vm_event_pause_count) )
> + vm_event_mark_and_pause(curr, ved);
>
> vm_event_ring_unlock(ved);
>
> --
> 1.9.1
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> https://lists.xen.org/xen-devel
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