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

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

On 1/16/24 16:16, Michael Brown wrote:
> On 16/01/2024 14:34, Laszlo Ersek wrote:
>> On 1/16/24 10:48, Michael Brown wrote:
>> IOW, my impression is that NestedInterruptTplLib can certainly handle
>> all scenarios thrown at it, but where it really matters is in the face
>> of an interrupt storm (not just "normal nesting"), and a storm is
>> unlikely (or even impossible?) on physical hardware.
>>
>> ... Oh, scratch that. "Interrupt storm" simply means that interrupts are
>> being delivered at a rate higher than the handler routine can service
>> them. IOW, the "storm" is not that interrupts are delivered *very
>> rapidly* in an absoulte sense. If interrupts are delivered at normal
>> frequency, but the handler is too slow to service *even that rate*, then
>> that also qualifies as "storm", because the nesting depth will *keep
>> growing*. It's not really the growth rate that matters; what matter is
>> the *trend*, i.e., the fact that there *is* growth (the stack gets
>> deeper and deeper). The stack might not overflow immediately, and if the
>> handler speeds up (for whatever reason), the stack might recover, but
>> there is nothing to prevent an overflow.
>>
>> So, in the end, I think you've convinced me.
> 
> :)
> 
>>> 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?
>>
>> Sounds like a valid idea to me.
>>
>> Could be greatly supported by a test case (to be run on the bare metal)
>> installing a slow handler that *eventually* exhausted the stack, when
>> not using NestedInterruptTplLib.
>>
>> (FWIW, IIRC, the UEFI spec warns about this -- it says something like,
>> "return from TPL_HIGH as soon as you can, otherwise the system will
>> become unstable".)
>>
>> Sorry for the wall of text, I find this very difficult to reason about.
> 
> I also find it very difficult to reason about, which is why
> NestedInterruptRestoreTpl() has 126 lines of comments providing a
> semi-formal proof of correctness for a mere 15 statements of C code!
> 
> In particular, I find it difficult to reason about when it would be safe
> for a platform to *not* use NestedInterruptTplLib.  It's clearly
> empirically difficult to trigger stack underflow via an interrupt
> "storm" on physical hardware, but I'm not convinced it's impossible.
> 
> I find it mentally easier to rely on the hard guarantee that
> NestedInterruptTplLib provides: that nested interrupts will continue to
> be delivered but that the number of interrupt-induced stack frames is
> bounded by the (small, finite) number of distinct TPL levels in existence.
> 
> 
> 
> While developing NestedInterruptTplLib, I did hack together a test case
> for a slow handler that would deliberately induce an interrupt storm,
> since I needed this to test that my code was working.  When triggered,
> this test would cause the machine to effectively hang due to servicing
> an endless storm of timer interrupts.  Before NestedInterruptTplLib, the
> stack would soon underflow and would typically cause a reboot (or other
> crash).  With NestedInterruptTplLib the machine would continue to
> service interrupts indefinitely.
> 
> How might such a test case be included in upstream EDK2?  I'm
> peripherally aware of EDK2 test infrastructure such as UEFI SCT, but
> I've never interacted with it yet.

I'm vaguely aware of a unit test framework inside edk2, but the best I
can give you is just this link:

https://github.com/tianocore/edk2/tree/master/UnitTestFrameworkPkg#unit-test-framework-package

There are some files under the directory "MdeModulePkg/Test" too;
git-log on that subdir, and perhaps the MdeModulePkg maintainers, might
provide more pointers.

The end of the readme linked above says to ask Bret, Mike and Sean, as well.

Laszlo



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