Re: [Qemu-devel] [PATCH V14 2/7] Add TPM (frontend) hardware interface (TPM TIS) to Qemu

[Stefan Berger]

On 02/21/2012 02:58 PM, Michael S. Tsirkin wrote:
> On Tue, Feb 21, 2012 at 10:05:26AM -0500, Stefan Berger wrote:
> > On 02/21/2012 07:18 AM, Michael S. Tsirkin wrote:
> > >
> > When the backend delivers the response it checks whether the
> > interface is used in interrupt mode and raises the interrupt.
> IMO it's the frontend that should send interrupts.
> Yes it kind of works for isa anyway, but e.g. pci
> needs to update configuration etc.

The code that causes the interrupt to be raised is in the frontend. The 
function doing that is invoked via callback from the backend. This 
should be ok?

> > The
> > backend enters the frontend code with a callback. In this function
> > also a signal is sent that may wake up the main thread that, upon
> > suspend, may be waiting for the last command to be processed and be
> > sleeping on a condition variable.
> >
> > I now added a function to the backend interface that is invoked by
> > the frontend to notify the backend of a TPM request. The backend
> > code can then either notify a thread (passthrough and libtpms
> > driver) or create a response right away and invoke that callback to
> > the front-end to deliver the response (null driver). How frontend
> > and backend handle notifications is isolated to the frontend and
> > backend with some backends (libtpms, passthough) sharing the code
> > for how the notification is done.
> >
> > Stefan
> Right. And all the locking/threading can then be internal to the backend.

Do you really want to replace code like this in the frontend:

qemu_mutex_lock(&s->state_lock)
[...]
qemu_mutex_unlock(&s->state_lock)


with


s->be_driver->ops->state_lock(s->be_driver)
[...]
s->be_driver->ops->state_unlock(s->be_driver))


where the backend starts protecting frontend data structures ?

At the moment there are two backends that need threading: the libtpms 
and passthrough backends. Both will require locking of datastructures 
that belong to the frontend. Only the null driver doesn't need a thread 
and the main thread can call into the backend, create the response and 
call via callback into the frontend to deliver the repsonse. If 
structures are protected via mutxes then only the NULL driver (which we 
don't want anyway) may end up grabbing mutexes that really aren't 
necessary while the two other backends need them. I don't see the 
mitextes as problematic. The frontend at least protects its data 
structures for the callbacks and other API calls it offers and they 
simply are thread-safe.

    Stefan