Re: [PATCH 0/2] KVM: SVM: Fix missing LAPIC TPR sync into VMCB::V_TPR with AVIC on

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 21, 2025, Alejandro Jimenez wrote:
> On 8/21/25 7:42 AM, Maciej S. Szmigiero wrote:
> > On 21.08.2025 10:18, Naveen N Rao wrote:
> > > > Yes, this breaks real guests when AVIC is enabled.
> > > > Specifically, the one OS that sometimes needs different handling and its
> > > > name begins with letter 'W'.
> > > 
> > > Indeed, Linux does not use TPR AFAIK.
> 
> I believe it does, 

Heh, yes, Linux technically "uses" the TPR in that it does a one-time write to
it.  But what Naveen really meant is that Linux doesn't actively use TPR to
manage what IRQs are masked/allowed, whereas Windows heavily uses TPR to do
exactly that.  Specifically, what matters is that Linux doesn't use TPR to _mask_
IRQs, and so clobbering it to '0' on migration is largely benign.

> during the local APIC initialization. When Maciej
> determined the root cause of this issue, I was wondering why we have not
> seen it earlier in Linux. I found that Linux takes a defensive approach and
> drains all pending interrupts during lapic initialization. Essentially, for
> each CPU, Linux will:
> - temporarily disable the Local APIC (via Spurious Int Vector Reg)
> - set the TPR to accept all "regular" interrupts i.e. tpr=0x10
> - drain all pending interrupts in ISR and/or IRR
> - attempt the above draining step a max of 512 times
> - then re-enable APIC and continue initialization
> 
> The relevant code is in setup_local_APIC()
> https://elixir.bootlin.com/linux/v6.16/source/arch/x86/kernel/apic/apic.c#L1533-L1545
> 
> So without Maciej's proposed change, other OSs that are not as resilient
> could also be affected by this issue.
> 
> Alejandro
> 
> > > - Naveen
> > > 
> > 
> > Thanks,
> > Maciej
> > 
> > 
>