Re: [edk2-devel] [PATCH 2/3] OvmfPkg: Add library to handle TPL from within nested interrupt handlers

["Michael Brown" <mcb30@ipxe.org>]

On 09/12/2022 15:02, Gerd Hoffmann wrote:
>> Add the Nested Interrupt TPL Library (NestedInterruptTplLib) to
>> provide helper functions that can be used by nested interrupt handlers
>> in place of RaiseTPL()/RestoreTPL().
> 
> So, in a nutshell this makes inner interrupt handler only run callbacks
> with a priority higher than the priority of the outer interrupt handler.
> Anything with equal or lower priority is deferred to the outer handler.

Essentially, yes, though for accuracy I would replace "run callbacks 
with" with "restore TPL to":

   The inner interrupt handler will only restore TPL to a priority
   higher than the priority of the outer interrupt handler. Anything
   with equal or lower priority is deferred to the outer handler.

The distinction matters because in the deferral case the inner interrupt 
will not run any callbacks at all: the running of callbacks is then 
deferred in its entirety to the outer interrupt handler (with a hard 
guarantee that the deferred running of callbacks will take place 
immediately).

> That way we never nest at the same priority, which in turn limits
> nesting to the number of different priorities in use.
> 
> Correct?

Yes, limiting nesting to the number of different priorities in use is 
exactly what this patch does.

> The idea looks sane to me and I couldn't spot any logic errors in this.

Great! :)

Thanks,

Michael