Re: [edk2-devel] [PATCH 1/6] UefiCpuPkg/LocalApicTimerDxe: Duplicate OvmfPkg/LocalApicTimerDxe driver

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

On 16/01/2024 08:47, Gerd Hoffmann wrote:
> I think the reason is that the next timer interrupt arriving while the
> handler is running still is *much* more likely in virtual machines
> because the vCPU does not get 100% of the of the physical CPU time
> slice.

The interrupt handler can run for an arbitrary length of time, because 
the call to RestoreTPL() can end up calling an application callback 
which in turn waits on further timer interrupts.

This is a legitimate use case within UEFI, so all timer interrupt 
handlers (not just in virtual machines) need to allow for the 
possibility that nested interrupts will occur.

> So on OVMF we will continue to need NestedInterruptTplLib.  I like the
> idea to have a Null version of the library, so OVMF does not need its
> own version of the driver just because of NestedInterruptTplLib.

I agree that there should not need to be two versions of LocalApicTimerDxe.

I would suggest moving NestedInterruptTplLib from OvmfPkg to MdeModulePkg.

The code is complex, but for the caller the complexity is completely 
hidden behind the calls to NestedInterruptRaiseTPL() and 
NestedInterruptRestoreTPL(), which straightforwardly replace what would 
otherwise be (unsafe) calls to RaiseTPL() and RestoreTPL(), as shown in

https://github.com/tianocore/edk2/commit/a086f4a#diff-1418ec21e24e4ce2f3d621113585e3bea11fbcfa3b77d54046e61be7928e0c92

For a new CPU architecture, the only requirement is to provide a short 
fragment (~5 lines) of code that can clear the interrupts-enabled flag 
in the EFI_SYSTEM_CONTEXT, as shown in 
OvmfPkg/Library/NestedInterruptTplLib/Iret.c.

I'm happy to send a patch to migrate NestedInterruptTplLib to 
MdeModulePkg, so that it can be consumed outside of OvmfPkg.  Shall I do 
this?

Thanks,

Michael



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