Re: [edk2-devel] [PATCH v3 5/5] MdeModulePkg: Extend NestedInterruptTplLib to support Arm CPUs

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

On 23/01/2024 15:51, Ard Biesheuvel wrote:
> On Tue, 23 Jan 2024 at 16:31, Michael Brown <mcb30@ipxe.org> wrote:
>> Add implementations of DisableInterruptsOnIret() for MDE_CPU_ARM and
>> MDE_CPU_AARCH64.  In both cases, the saved IRQs-disabled and
>> FIQs-disabled flags are set in the stored processor status register
>> (matching the behaviour of DisableInterrupts(), which also sets both
>> flags).
> 
> Thanks for this.
> 
> You can drop the FIQ bits, though: anything that can run Tianocore
> will have the FIQs routed to the secure world, and all of the higher
> level en/disable interrupt code only reasons about the IRQ line.

Thank you.

My commit message was incorrect, sorry: DisableInterrupts() in MdePkg 
does indeed disable only IRQs.  I was accidentally looking at 
ArmDisableInterrupts() in ArmPkg, which disables both IRQs and FIQs.

My (incomplete) understanding of Arm behaviour is that when an interrupt 
is raised, both IRQs and FIQs will be disabled by hardware.  When the 
interrupt handler ends up enabling interrupts during RestoreTPL(), only 
the IRQs will be re-enabled since EnableInterrupts() touches only the 
IRQ bit.  This state can persist for an extended period of time (e.g. 30 
seconds) if a callback at TPL_CALLBACK ends up waiting for further timer 
events.

Q1: Is it a problem to have this situation, with FIQs disabled for an 
extended period of time?

Q2: What happens with FIQs when there is no secure world (e.g. using 
ArmVirtQemu with "-M virt")?

Q3: Would you like me to add in the extra patch that modifies TimerDxe 
to use NestedInterruptTplLib?

Many thanks,

Michael



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