[PATCH v2] KVM: nSVM: Preserve registers modifications done before nested_svm_vmexit()

[PATCH v2] KVM: nSVM: Preserve registers modifications done before nested_svm_vmexit()

[Vitaly Kuznetsov]

L2 guest hang is observed after 'exit_required' was dropped and nSVM
switched to check_nested_events() completely. The hang is a busy loop when
e.g. KVM is emulating an instruction (e.g. L2 is accessing MMIO space and
we drop to userspace). After nested_svm_vmexit() and when L1 is doing VMRUN
nested guest's RIP is not advanced so KVM goes into emulating the same
instruction which caused nested_svm_vmexit() and the loop continues.

nested_svm_vmexit() is not new, however, with check_nested_events() we're
now calling it later than before. In case by that time KVM has modified
register state we may pick stale values from VMCB when trying to save
nested guest state to nested VMCB.

nVMX code handles this case correctly: sync_vmcs02_to_vmcs12() called from
nested_vmx_vmexit() does e.g 'vmcs12->guest_rip = kvm_rip_read(vcpu)' and
this ensures KVM-made modifications are preserved. Do the same for nSVM.

Generally, nested_vmx_vmexit()/nested_svm_vmexit() need to pick up all
nested guest state modifications done by KVM after vmexit. It would be
great to find a way to express this in a way which would not require to
manually track these changes, e.g. nested_{vmcb,vmcs}_get_field().

Co-debugged-with: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
- Changes from v1:
 s,nVMX,nSVM,g + typos [Sean Christopherson]

- To certain extent we're fixing currently-pending 'KVM: SVM: immediately
 inject INTR vmexit' commit but I'm not certain about that. We had so many
 problems with nested events before switching to check_nested_events() that
 what worked before could just be treated as a miracle. Miracles tend to
 appear and disappear all of a sudden.
---
 arch/x86/kvm/svm/nested.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 0f02521550b9..6b1049148c1b 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -537,9 +537,9 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
 	nested_vmcb->save.cr2    = vmcb->save.cr2;
 	nested_vmcb->save.cr4    = svm->vcpu.arch.cr4;
 	nested_vmcb->save.rflags = kvm_get_rflags(&svm->vcpu);
-	nested_vmcb->save.rip    = vmcb->save.rip;
-	nested_vmcb->save.rsp    = vmcb->save.rsp;
-	nested_vmcb->save.rax    = vmcb->save.rax;
+	nested_vmcb->save.rip    = kvm_rip_read(&svm->vcpu);
+	nested_vmcb->save.rsp    = kvm_rsp_read(&svm->vcpu);
+	nested_vmcb->save.rax    = kvm_rax_read(&svm->vcpu);
 	nested_vmcb->save.dr7    = vmcb->save.dr7;
 	nested_vmcb->save.dr6    = svm->vcpu.arch.dr6;
 	nested_vmcb->save.cpl    = vmcb->save.cpl;
-- 
2.25.4

Re: [PATCH v2] KVM: nSVM: Preserve registers modifications done before nested_svm_vmexit()

[Paolo Bonzini]

On 27/05/20 11:01, Vitaly Kuznetsov wrote:
> L2 guest hang is observed after 'exit_required' was dropped and nSVM
> switched to check_nested_events() completely. The hang is a busy loop when
> e.g. KVM is emulating an instruction (e.g. L2 is accessing MMIO space and
> we drop to userspace). After nested_svm_vmexit() and when L1 is doing VMRUN
> nested guest's RIP is not advanced so KVM goes into emulating the same
> instruction which caused nested_svm_vmexit() and the loop continues.
> 
> nested_svm_vmexit() is not new, however, with check_nested_events() we're
> now calling it later than before. In case by that time KVM has modified
> register state we may pick stale values from VMCB when trying to save
> nested guest state to nested VMCB.
> 
> nVMX code handles this case correctly: sync_vmcs02_to_vmcs12() called from
> nested_vmx_vmexit() does e.g 'vmcs12->guest_rip = kvm_rip_read(vcpu)' and
> this ensures KVM-made modifications are preserved. Do the same for nSVM.
> 
> Generally, nested_vmx_vmexit()/nested_svm_vmexit() need to pick up all
> nested guest state modifications done by KVM after vmexit. It would be
> great to find a way to express this in a way which would not require to
> manually track these changes, e.g. nested_{vmcb,vmcs}_get_field().
> 
> Co-debugged-with: Paolo Bonzini <pbonzini@redhat.com>

Huh, not really, but thanks. :)  Patch queued!

Paolo