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

["Laszlo Ersek" <lersek@redhat.com>]

On 1/15/24 19:10, Kinney, Michael D wrote:
> Hi Ray,
>
> I think nesting may be possible in physical platforms, but very hard
> to induce.
>
> One option is to consolidate to a single LocalApicTimerDxe
> implementation in the UefiCpuPkg, but allow the platform DSC to either
> specify a Null NestedInterruptTplLib for physical platforms or the
> full one from the OvmfPkg for VM use cases.
>
> The other changes could be included for OvmfPkg use cases.  If the VM
> does not support the CPUID leafs, then the PCD value should be used.

All of this sounds great!

(And I do think that *some* nesting should not be a problem, on either
physical or virtual platforms, as (IIRC) the interrupt handler stack can
accommodate a certain level of reentrancy. It's the "storm" that is a
problem, but that should never occur on physical machines, I reckon.)

Assuming a v2 is coming up, my advance request would be for Ray to
re-review the math in patch #4, before posting v2, focusing on integer
overflows, and SafeIntLib (if needed). I've not looked at the patch in
detail yet, but I reviewed something similar not so long ago [1]. The
latter frequency calculation code had been quite commonly used in edk2,
and I needed hours to review just that one patch. Plus, the review
turned up a number of issues to fix. So, preferably such a patch should
not only prevent any integer overflows, but also clarify, in comments,
why overflows are impossible, and/or how they are prevented.

[1] https://edk2.groups.io/g/devel/message/111613
    http://mid.mail-archive.com/2e42db7c-a927-f47b-7d80-632895b11e1b@redhat.com

Thanks!
Laszlo



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