Re: [Qemu-devel] [PATCH] kvm: First step to push iothread lock out of inner run loop

[Marcelo Tosatti]

On Thu, Jun 28, 2012 at 09:10:39AM -0500, Anthony Liguori wrote:
> >
> >1.  read_lock(memmap_lock)
> >2.  MemoryRegionSection mrs = lookup(addr)
> >3.  qom_ref(mrs.mr->dev)
> >4.  read_unlock(memmap_lock)
> >
> >5.  mutex_lock(dev->lock)
> >6.  dispatch(&mrs, addr, data, size)
> >7.  mutex_unlock(dev->lock)
> 
> Just a detail, I don't think we should acquire a device specific
> lock in global code.  Rather, I think we should acquire the global
> lock before dispatch unless a MemoryRegion is marked as being
> unlocked.

"The basic plan is introduce granular locking starting at the KVM
dispatch level until we can get to MemoryRegion dispatch. We'll then
have some way to indicate that a MemoryRegion's callbacks should be
invoked without holding the qemu global mutex."

Before that is possible, the callback must not make use of data
structures currently protected by qemu_global_mutex, such as 
timers, interrupts (that is, you would have to split locks 
for each individual service invoked from inside callbacks,
which is a recipe for disaster).

With lock_device() below you can have

mutex_lock(dev->lock)
device specific work
mutex_lock(qemu_global_mutex)
raise irq, send packet, etc
mutex_unlock(qemu_global_mutex)
mutex_unlock(dev->lock)

and iothread doing the select() pseudocode in the previous email.