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

[Andrew Fish <afish@xxxxxxxxx>]

> On Sep 16, 2019, at 10:36 AM, Laszlo Ersek <lersek@xxxxxxxxxx> wrote:
> 
> 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.
> 

Laszlo,

I agree given the timer event activity is stopped prior to calling any of the 
EXIT_BOOT_SERVICES events there is usually not a requirement to call the 
drivers Stop() function. Generally Exit Boot Services events just turn off DMA, 
or any other hardware that could touch memory that is being freed back for OS 
usage. Since the timer activity, and thus all event activity is stopped there 
is not a lot of ways for the drivers to ever have any EFI code run again. 

The only other exception I can think of is if the OS driver makes some kind of 
assumption about the state of the hardware.

Thanks,

Andrew Fish


> 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
> DriverBindingStop().
> 
> - 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.)
> 
> Thanks
> Laszlo
> 
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