Re: [PATCH 4/6] kvm tools: Add rwlock wrapper

[Avi Kivity <avi@redhat.com>]

On 05/28/2011 10:45 PM, Sasha Levin wrote:
> On Sat, 2011-05-28 at 17:24 +0200, Ingo Molnar wrote:
> >  * Sasha Levin<levinsasha928@gmail.com>  wrote:
> >
> >  >  So the basic plan here is to allocate a futex(?) for each VCPU
> >  >  thread, and have the writer thread lock all futexes when it needs
> >  >  to write?
> >  >
> >  >  If we assume we only have one writer thread, it can stay pretty
> >  >  simple.
> >
> >  We can use an even simpler and more scalable method:
> >
> >    - writers can 'stop' all other threads, by sending them a
> >      threadpool signal and waiting for each thread to have completed
> >      processing their current work and notifying the writer back that
> >      they have stopped running.
> >
> >  This means that the read-side lock is _zero instructions_, basically
> >  just a barrier() to make sure the compiler does not move instructions
> >  across threadpool functions (it wont).
> >
> >  This method requires that we know about every worker thread - i.e.
> >  no-one does a stray pthread_create() and uses data structures from
> >  there. It also requires that each worker thread can 'stop' within a
> >  reasonable amount of time.
>
> In this case, maybe instead of implementing it as a 'lock', we can
> implement it as a way to stop all vcpu threads from reentering the
> kernel (KVM_RUN):
>
> 1. Set a 'vcpu-stop' flag.
> 2. Signal all VCPUs to exit KVM_RUN.
> 3. VCPU threads now wait on our lock before reentering into KVM_RUN -
> the writer thread waits until waiting threads = VCPU count.
> 4. Writer thread writes, releases lock.
>
> So instead of it being a lock in MMIO, IO-ports, etc - it's a method to
> stop the entire guest which could be used during configuration updates
> (and anything else we might think of). It could also be used as a method
> for users to 'pause' the guest.

Yes, this is equivalent to the kernel's stop_machine_run().  It's a 
heavyweight method but it should work just fine.

-- 
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signature is too narrow to contain.