Re: [PATCH v2 0/4] KVM: x86: APICv cleanups

Maxim Levitsky
2021-10-22T14:57:10Z

On Fri, 2021-10-22 at 12:12 +0200, Paolo Bonzini wrote:
> On 22/10/21 02:49, Sean Christopherson wrote:
> > APICv cleanups and a dissertation on handling concurrent APIC access page
> > faults and APICv inhibit updates.
> >
> > I've tested this but haven't hammered the AVIC stuff, I'd appreciate it if
> > someone with the Hyper-V setup can beat on the AVIC toggling.
> >
> > Sean Christopherson (4):
> > KVM: x86/mmu: Use vCPU's APICv status when handling APIC_ACCESS
> > memslot
> > KVM: x86: Move SVM's APICv sanity check to common x86
> > KVM: x86: Move apicv_active flag from vCPU to in-kernel local APIC
> > KVM: x86: Use rw_semaphore for APICv lock to allow vCPU parallelism
> >
> > arch/x86/include/asm/kvm_host.h | 3 +-
> > arch/x86/kvm/hyperv.c | 4 +--
> > arch/x86/kvm/lapic.c | 46 ++++++++++---------------
> > arch/x86/kvm/lapic.h | 5 +--
> > arch/x86/kvm/mmu/mmu.c | 29 ++++++++++++++--
> > arch/x86/kvm/svm/avic.c | 2 +-
> > arch/x86/kvm/svm/svm.c | 2 --
> > arch/x86/kvm/vmx/vmx.c | 4 +--
> > arch/x86/kvm/x86.c | 59 ++++++++++++++++++++++-----------
> > 9 files changed, 93 insertions(+), 61 deletions(-)
> >
>
> Queued, thanks. I only made small edits to the comment in patch
> 1, to make it very slightly shorter.
>
> * 2a. APICv is globally disabled but locally enabled, and this
> * vCPU acquires mmu_lock before __kvm_request_apicv_update
> * calls kvm_zap_gfn_range(). This vCPU will install a stale
> * SPTE, but no one will consume it as (a) no vCPUs can be
> * running due to the kick from KVM_REQ_APICV_UPDATE, and
> * (b) because KVM_REQ_APICV_UPDATE is raised before the VM
> * state is update, vCPUs attempting to service the request
> * will block on apicv_update_lock. The update flow will
> * then zap the SPTE and release the lock.
>
> Paolo
>

Hi Paolo and Sean!

Could you expalain to me why the scenario when I expalined about in my reply previous version of patch 1
is not correct?

This is the scenario I was worried about:

vCPU0 vCPU1
===== =====

- disable AVIC
- VMRUN
- #NPT on AVIC MMIO access
- *stuck on something prior to the page fault code*
- enable AVIC
- VMRUN
- *still stuck on something prior to the page fault code*

- disable AVIC:

- raise KVM_REQ_APICV_UPDATE request

- set global avic state to disable

- zap the SPTE (does nothing, doesn't race
with anything either)

- handle KVM_REQ_APICV_UPDATE -
- disable vCPU0 AVIC

- VMRUN
- *still stuck on something prior to the page fault code*

...
...
...

- now vCPU1 finally starts running the page fault code.

- vCPU1 AVIC is still enabled
(because vCPU1 never handled KVM_REQ_APICV_UPDATE),
so the page fault code will populate the SPTE.

- handle KVM_REQ_APICV_UPDATE
- finally disable vCPU1 AVIC

- VMRUN (vCPU1 AVIC disabled, SPTE populated)

***boom***

Best regards,
Maxim Levitsky