Processor Delays with Locking in multi-core VMs

[Penny_Svenkeson]

Hi all,

In our hypervisor implementation, we can have multiple cores assigned to a single VM.  In the multi-core VM configuration, we are seeing longer delays under virtualization for cores getting locks (even when there is no contention for the lock cell).

If a core (or thread) has a guest cache line resident and takes a VMEXIT for some reason and another core want to get ownership of the guest cache line (for atomic operation), does the 2^nd core have to wait for the VMRESUME on the first core before getting ownership?

Are there any other reasons that would prevent a core from completing an atomic operation if other cores are in a VMEXIT condition?

Core1 has cache line A

Core1 takes a VMEXIT for timer interrupt

Core2 tries a sync_fetch_and_add to cache line A    <-- get ownership here

Core1 does a VMRESUME                                     <-- or wait till here for ownership

Thanks for any insight.

Penny

[Quoc-Thai_L_Intel]

Hi Penny,
Here is a feedback from my colleague:
"There is nothing related to VMEXIT that would prevent normal cache snoop and response; so, no, the requester would not be delayed from getting ownership of the line."
-Thai