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

[Michael S. Tsirkin]

On Tue, Sep 17, 2013 at 07:16:15PM +0200, Paolo Bonzini wrote:
> Il 17/09/2013 19:07, Michael S. Tsirkin ha scritto:
> > After memory_region_del_subregion returns,
> > it's a promise that there will not be accesses
> > to the region.
> 
> It's racy anyway.  You can have memory_region_del_subregion happen one
> clock cycle after the other (physical) CPU has done checked that there
> will not be accesses to the region.

Fix it then. Stick synchronize_rcu() in memory_region_del_subregion.

Are you trying to convince me there's no way to have
synchronous APIs in presence of RCU?

> A real bus has a "big PCI lock" (there can be only one transaction at a
> time), which is exactly what we want to get rid of.

Not really, in a bridged setup transactions on the
secondary bus are handled in parallel with transactions
on the primary bus.

Also, there are split transactions completed
asynchronously: guests that care about ordering
do flushes e.g. using memory reads.
The logic is in the device/memory region really.

> > So I'm not even sure we really need to move destroy to finalize anymore ...
> 
> We definitely need to move it.  Even if we added a flag that we set in
> memory_region_del_subregion, we need to check it later when AIO completes.
> 
> Paolo

Flag where? region can become subregion of another
region entirely.

The solution for RCU is to flush on changes, not add
more flags and locks.

-- 
MST