Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 02/19/2014 06:12 AM, Waiman Long wrote:
> On 02/18/2014 04:28 PM, Peter Zijlstra wrote:
>> On Tue, Feb 18, 2014 at 02:30:12PM -0500, Waiman Long wrote:
>>> I will start looking at how to make it work with paravirt. Hopefully, it
>>> won't take too long.
>> The cheap way out is to simply switch to the test-and-set spinlock on
>> whatever X86_FEATURE_ indicates a guest I suppose.
>
> I don't think there is X86_FEATURE flag that indicates running in a
> guest. In fact, a guest should never find out if it is running virtualized.
>
> After reading the current PV ticketlock implementation, I have a rough
> idea of what I need to do to implement PV support in qspinlock. A large
> portion of PV ticketlock code is find out the CPU number of the next one
> to get the lock. The current qspinlock implementation has already
> included CPU number of the previous member in the queue and it should be
> pretty easy to store CPU number of the next one in the queue node
> structure. These CPU numbers can then be supplied to the kick_cpu()
> function to schedule in the require the CPU to make sure that progress
> can be made.

That is correct.
Strict serialization of the lock is usually a headache for virtualized
guest (especially when overcommitted). I am eager to test the next
version.