Re: [Qemu-devel] [PATCH v2 00/38] Delay destruction of memory regions to instance_finalize

[Michael S. Tsirkin]

On Wed, Sep 18, 2013 at 09:40:19AM +0200, Paolo Bonzini wrote:
> Il 18/09/2013 07:48, Michael S. Tsirkin ha scritto:
> > So I think the fix is actually obeying ordering rules,
> > that is know that write is in progress
> > and flush on read.
> 
> I think this can be modeled as a generic, synchronous
> (*busmaster_cancel)(PCIDevice*) callback, that is called after bus
> master is turned off.  You don't even really have to wait for a read.

Not really.
Bus master is just an single instance of the bigger issue.
It could be any device-specific register just as well.

PCI reads and writes must obey ordering rules.
ATM MMIO and DMA achieve this by using a single lock.
If you want to move MMIO and DMA out of a common lock you must find some
other way to force ordering.

> > I think moving memory region destroy out to finalize makes sense
> > irrespectively, as long as destroy is made idempotent so we can simply
> > destroy everything without worrying whether we initialized it.
> > 
> > The rest of the changes will be harder, we'll have to do
> > them carefully on a case by case basis.
> 
> Good, we are in agreement then.
> 
> Paolo