Re: [edk2-devel] RFC: Another solution to the nested interrupt issue

["Ni, Ray" <ray.ni@intel.com>]

> On 25/01/2024 07:57, Ni, Ray wrote:
> > This mail is to bring another approach to solve the stack-overflow due
> > to nested interrupts. Michael solved this problem in OVMF through
> > NestedInterruptTplLib.
> >
> > I made a draft patch as attached “DxeCore.diff”. The patch simply to
> > avoid the interrupt in enable state when TPL is dropped to the
> > interrupted TPL. The interrupt will be enabled later through “IRET”.
> 
> I don't disagree with the approach, but it does break the API as per the
> UEFI PI specification (version 1.8 section II-12.10), and so this is not
> something that can just be dropped in as an EDK2 code change.

You think that the TimerInterruptHandler() doesn't raise/restore TPL
which would violate the PI spec as PI spec says " NotifyFunction ... executes at EFI_TPL_HIGH_LEVEL."?

I do not think the PI spec requires TimerInterruptHandler() raises TPL
to HIGH before invoking NotifyFunction. It just means the NotifyFunction
will execute at TPL_HIGH.

If you review HpetTimer driver, it does not raise TPL to HIGH before
invoking NotifyFunction.

And I think implementing the DxeCore changes as attached does not
prevent the TimerInterruptHandler() from calling raise/restore TPL.

So, with the changes done in DxeCore, a timer driver could either
not raise/restore TPL in TimerInterruptHandler(), or it calls
NestedInterruptTplLib if it wants.

I like the idea behind NestedInterruptTplLib. The only concern is
the maintenance effort. I want to avoid the difficulty of debuggability
brought by NestedInterruptTplLib when some timer interrupt related
issues happen in future. 

I am ok with OvmfPkg code using NestedInterruptTplLib.

Thanks,
Ray



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