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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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