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

Lendacky, Thomas

On 5/11/22 19:52, Min Xu via 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:
00020 ISR:

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.



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