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?

Thanks
Min 


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