Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock
From: Waiman Long
Date: Thu Feb 20 2014 - 12:37:33 EST
On 02/19/2014 02:48 PM, Peter Zijlstra wrote:
On Wed, Feb 19, 2014 at 02:24:49PM -0500, Waiman Long wrote:
On 02/19/2014 03:51 AM, Peter Zijlstra wrote:
On Tue, Feb 18, 2014 at 07:42:20PM -0500, 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.
No it very much should; how else is paravirt ever going to work?
We do have a CONFIG_PARAVIRT macro that turns on or off PV support. The
queue spinlock can be easily changed into an unfair lock which allows lock
stealing. We could have a config option to make it unfair in the PARAVIRT
environment, but I don't think Linus like the idea of an unfair lock.
No; a guest is very much aware of paravirt. See for example the
static_key_false(¶virt_ticketlocks_enabled). It would be impossible
to set that branch if you never knew you were a guest.
Yes, that is true for paravirt, but I was talking about virtualization
in general.
-Longman
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