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Re: [Xen-devel] [edk2-devel] [PATCH 09/11] OvmfPkg/XenBusDxe: Fix NotifyExitBoot to avoid Memory Allocation Services

On 09/13/19 16:50, Anthony PERARD wrote:
> This patch fix the EVT_SIGNAL_EXIT_BOOT_SERVICES handler to avoid
> using the Memory Allocation Services.
> This comes with a new interface named RegisterExitCallback so that PV
> drivers can disconnect from the backend before XenBusDxe is teared
> down.
> Instead of using Disconnect() to tear down the XenBus driver and the
> children drivers, we are going to ask every driver using
> XENBUS_PROTOCOL to disconnect from the hardware via the callback set
> with RegisterExitCallback, then reset the xenstore shared ring and
> the grant table.

I think this approach -- a lower-level bus driver calling out to
dependent device drivers -- is quite unusual.

How about the following instead:

- introduce two XenBusIo protocol member functions, AddReference() and
RemoveReference(). RemoveReference() should take a BOOLEAN called
"HandOffToOs". The device drivers should call AddReference() just before
exiting DriverBindingStart() with success, and RemoveReference(FALSE) in

- these protocol member functions would increment / decrement a
reference counter in the underlying XenBus abstraction. Additionally,
RemoveReference() would store the HandOffToOs parameter to a bus-level
BOOLEAN too (regardless of previous value stored there -- a TRUE->FALSE
transition would never happen anyway; see below).

- both XenBusDxe and the Xen device drivers should register EBS
callbacks, per controller handle (in BindingStart()), and unregister
them (in BindingStop())

- the ordering between EBS notification functions (queued at the same
TPL) is unspecified. In the device driver notification functions, the
last action should be a call to XenBusIo->RemoveReference(TRUE) -- after
the device-specific "forget me" actions have been done.

- if RemoveReference() gets a TRUE parameter, then it should check the
resultant (post-decrement) value of the refcount. If it has gone to
zero, RemoveReference() should re-set the xenbus / xenstore connection.
If the parameter is FALSE, it shouldn't do anything particular after
decrementing the refcount.

- in the XenBus EBS handler, if the refcount is positive at the time of
the call, nothing should be done. Otherwise, if HandOffToOs is TRUE,
nothing should be done, similarly. Otherwise, the xenbus/xenstore
connection should be re-set.

The idea is that normal Start/Stop should manage the refcount as
expected. At ExitBootServices(), the XenBus level handler should only
clear the connection to the hypervisor if no RemoveReference() call has
done, or will do, it. (If the counter is positive, then a later
RemoveReference() call will do it; if it's zero but HandOffToOs is TRUE,
then it's been done already. If the counter is zero and the BOOLEAN is
FALSE, then all devices have been disconnected normally with Stop() --
or none have been connected at all --, before ExitBootServices(), so the
XenBus driver itself has to ask for being forgotten.)

Admittedly, this is more complicated (due to the unspecified ordering
between EBS notifications). I just feel it's more idiomatic to go
through normal protocol member functions in EBS notification functions,
rather than special callbacks.

(Side comment: the reference counting could normally be replaced by
gBS->OpenProtocolInformation(); however, that service itself allocates
memory, so we can't use it in EBS notification functions.)


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