Re: [PATCH 0/2] KVM: x86: Fix and cleanup for recent AVIC changes

[Sean Christopherson <seanjc@google.com>]

On Sun, Oct 10, 2021, Maxim Levitsky wrote:
> On Fri, 2021-10-08 at 18:01 -0700, Sean Christopherson wrote:
> > Belated "code review" for Maxim's recent series to rework the AVIC inhibit
> > code.  Using the global APICv status in the page fault path is wrong as
> > the correct status is always the vCPU's, since that status is accurate
> > with respect to the time of the page fault.  In a similar vein, the code
> > to change the inhibit can be cleaned up since KVM can't rely on ordering
> > between the update and the request for anything except consumers of the
> > request.
> > 
> > Sean Christopherson (2):
> >   KVM: x86/mmu: Use vCPU's APICv status when handling APIC_ACCESS
> >     memslot
> >   KVM: x86: Simplify APICv update request logic
> > 
> >  arch/x86/kvm/mmu/mmu.c |  2 +-
> >  arch/x86/kvm/x86.c     | 16 +++++++---------
> >  2 files changed, 8 insertions(+), 10 deletions(-)
> > 
> 
> Are you sure about it? Let me explain how the algorithm works:
> 
> - kvm_request_apicv_update:
> 
> 	- take kvm->arch.apicv_update_lock
> 
> 	- if inhibition state doesn't really change (kvm->arch.apicv_inhibit_reasons still zero or non zero)
> 		- update kvm->arch.apicv_inhibit_reasons
> 		- release the lock
> 
> 	- raise KVM_REQ_APICV_UPDATE
> 		* since kvm->arch.apicv_update_lock is taken, all vCPUs will be
> 		kicked out of guest mode and will be either doing someing in
> 		the KVM (like page fault) or stuck on trying to process that
> 		request the important thing is that no vCPU will be able to get
> 		back to the guest mode.
> 
> 	- update the kvm->arch.apicv_inhibit_reasons
> 		* since we hold vm->arch.apicv_update_lock vcpus can't see the new value

This assertion is incorrect, kvm_apicv_activated() is not guarded by the lock.

> 	- update the SPTE that covers the APIC's mmio window:

This won't affect in-flight page faults.


   vCPU0                               vCPU1
   =====                               =====
   Disabled APICv
   #NPT                                Acquire apicv_update_lock
                                       Re-enable APICv
   kvm_apicv_activated() == false
   incorrectly handle as regular MMIO
                                       zap APIC pages
   MMIO cache has bad entry