Re: [edk2-devel] [PATCH V7 36/37] UefiCpuPkg: Setting initial-count register as the last step

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 5/11/22 19:52, Min Xu via groups.io wrote:
> On May 11, 2022 10:06 PM, Lendacky, Thomas wrote:
>> On 5/10/22 21:00, Xu, Min M wrote:
>>> On May 11, 2022 4:30 AM, Tom Lendacky wrote:
>>>> I'm replying to this patch since I can't find patch V12 46/47
>>>> anywhere in my email.
>>>>
>>>> I've bisected a regression in the Linux kernel to this patch when an
>>>> SEV-SNP guest is booted. The following message is issued in the
>>>> kernel for every AP being brought online:
>>>>
>>>> APIC: Stale IRR:
>>>>
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,000
>>>> 00020 ISR:
>>>>
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,000
>>>> 00000
>>>>
>>>> Possibly a timing issue involving the mode switch with the interrupt
>>>> unmasked. If I leave the interrupt masked and only un-mask it after
>>>> the programming of the init-count, then the message goes away.
>>>
>>> Do you mean in InitializeApicTimer, it should follow below steps:
>>> 1. mask LvtTimer. (set LvtTimer.Bits.Mask = 1) 2. Do other stuff,
>>> including programing the init-count register.
>>> 3. un-mask LvtTimer (set LvtTimer.Bit.Mask = 0)
>>
>> Yes, I believe so. I'm not an expert on the APIC timer, but that seems
>> reasonable to me.
> I tested this fix in Td guest and it has no side effect.
> I check the Intel SDM (Vol.3A Chap 10.5 Handling Local Interrupts) but it doesn't describe the actual sequence of LvtTimer.Bits.Mask and  programming of init-count register.
> @ Ni, Ray,  What's your thought about it?

I guess you can theoretically miss an interrupt if your initial count is 
expires before you unmask the interrupt, so I think your fix is correct 
and no changes are needed.

I need to double check whether I'm properly resetting the APIC when APs 
are booted multiple times. Since this only occurs with SNP, I think this 
is on my end.

Thanks,
Tom

> 
> Thanks
> Min
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