Re: [Xen-devel] [PATCH] common/vm_event: Prevent guest locking with large max_vcpus

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 02/08/2017 07:20 PM, Andrew Cooper wrote:
> On 08/02/17 17:08, Razvan Cojocaru wrote:
>> On 02/08/2017 06:25 PM, Tamas K Lengyel wrote:
>>>
>>> On Wed, Feb 8, 2017 at 2:00 AM, Razvan Cojocaru
>>> <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>> 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.
>>>
>>>     Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx
>>>     <mailto:rcojocaru@xxxxxxxxxxxxxxx>>
>>>     ---
>>>      xen/common/vm_event.c | 3 ++-
>>>      1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>>     diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
>>>     index 82ce8f1..2005a64 100644
>>>     --- a/xen/common/vm_event.c
>>>     +++ b/xen/common/vm_event.c
>>>     @@ -316,7 +316,8 @@ 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 )
>>>     +    if( current->domain == d && avail_req < d->max_vcpus &&
>>>     +        !atomic_read( &current->vm_event_pause_count ) )
>>>              vm_event_mark_and_pause(current, ved);
>>>
>>>
>>> Hi Razvan,
>>> I would also like to have the change made in this patch that unblocks
>>> the vCPUs as soon as a spot opens up on the ring. Doing just what this
>>> patch has will not solve the problem if there are asynchronous events used.
>> Fair enough, I thought that might need more discussion and thus put into
>> a subsequent patch, but I'll modify that as well, give it a spin and
>> submit V2.
> 
> If you are spinning a v2, please pull current out into a variable at the
> start (more efficient), and atomic_read() doesn't need spaces inside.

Will do.


Thanks,
Razvan

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