Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support forlinux guests running on KVM hypervisor
From: Raghavendra K T
Date: Wed Jul 17 2013 - 10:35:42 EST
On 07/17/2013 07:43 PM, Raghavendra K T wrote:
On 07/17/2013 06:55 PM, Gleb Natapov wrote:
On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote:
On 07/17/2013 06:15 PM, Gleb Natapov wrote:
On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote:
Instead of halt we started with a sleep hypercall in those
versions. Changed to halt() once Avi suggested to reuse
existing sleep.
If we use older hypercall with few changes like below:
kvm_pv_wait_for_kick_op(flags, vcpu, w->lock )
{
// a0 reserved for flags
if (!w->lock)
return;
DEFINE_WAIT
...
end_wait
}
How would this help if NMI takes lock in critical section. The thing
that may happen is that lock_waiting->want may have NMI lock
value, but
lock_waiting->lock will point to non NMI lock. Setting of want and
lock
have to be atomic.
True. so we are here
non NMI lock(a)
w->lock = NULL;
smp_wmb();
w->want = want;
NMI
<---------------------
NMI lock(b)
w->lock = NULL;
smp_wmb();
w->want = want;
smp_wmb();
w->lock = lock;
---------------------->
smp_wmb();
w->lock = lock;
so how about fixing like this?
again:
w->lock = NULL;
smp_wmb();
w->want = want;
smp_wmb();
w->lock = lock;
if (!lock || w->want != want) goto again;
NMI can happen after the if() but before halt and the same situation
we are trying to prevent with IRQs will occur.
True, we can not fix that. I thought to fix the inconsistency of
lock,want pair.
But NMI could happen after the first OR condition also.
/me thinks again
lock_spinning() can check that it is called in nmi context and bail out.
Good point.
I think we can check for even irq context and bailout so that in irq
context we continue spinning instead of slowpath. no ?
How often this will happens anyway.
I know NMIs occur frequently with watchdogs. or used by sysrq-trigger
etc.. But I am not an expert how frequent it is otherwise.
Forgot to ask if Peter has any points on NMI frequency.
But even
then if they do not use spinlock, we have no problem as already pointed.
I can measure with debugfs counter how often it happens.
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