Re: [PATCH v2 0/7] KVM: nVMX: Fixes for nested state migration when eVMCS is in use

From: Maxim Levitsky
Date: Mon May 24 2021 - 08:09:10 EST

On Mon, 2021-05-17 at 15:50 +0200, Vitaly Kuznetsov wrote:
> Changes since v1 (Sean):
> - Drop now-unneeded curly braces in nested_sync_vmcs12_to_shadow().
> - Pass 'evmcs->hv_clean_fields' instead of 'bool from_vmentry' to
> copy_enlightened_to_vmcs12().
> Commit f5c7e8425f18 ("KVM: nVMX: Always make an attempt to map eVMCS after
> migration") fixed the most obvious reason why Hyper-V on KVM (e.g. Win10
> + WSL2) was crashing immediately after migration. It was also reported
> that we have more issues to fix as, while the failure rate was lowered
> signifincatly, it was still possible to observe crashes after several
> dozens of migration. Turns out, the issue arises when we manage to issue
> KVM_GET_NESTED_STATE right after L2->L2 VMEXIT but before L1 gets a chance
> to run. This state is tracked with 'need_vmcs12_to_shadow_sync' flag but
> the flag itself is not part of saved nested state. A few other less
> significant issues are fixed along the way.
> While there's no proof this series fixes all eVMCS related problems,
> Win10+WSL2 was able to survive 3333 (thanks, Max!) migrations without
> crashing in testing.
> Patches are based on the current kvm/next tree.
> Vitaly Kuznetsov (7):
> KVM: nVMX: Introduce nested_evmcs_is_used()
> KVM: nVMX: Release enlightened VMCS on VMCLEAR
> KVM: nVMX: Ignore 'hv_clean_fields' data when eVMCS data is copied in
> vmx_get_nested_state()
> KVM: nVMX: Force enlightened VMCS sync from nested_vmx_failValid()
> KVM: nVMX: Reset eVMCS clean fields data from prepare_vmcs02()
> KVM: nVMX: Request to sync eVMCS from VMCS12 after migration
> KVM: selftests: evmcs_test: Test that KVM_STATE_NESTED_EVMCS is never
> lost
> arch/x86/kvm/vmx/nested.c | 110 ++++++++++++------
> .../testing/selftests/kvm/x86_64/evmcs_test.c | 64 +++++-----
> 2 files changed, 115 insertions(+), 59 deletions(-)

Hi Vitaly!

In addition to the review of this patch series, I would like
to share an idea on how to avoid the hack of mapping the evmcs
in nested_vmx_vmexit, because I think I found a possible generic
solution to this and similar issues:

The solution is to always set nested_run_pending after
nested migration (which means that we won't really
need to migrate this flag anymore).

I was thinking a lot about it and I think that there is no downside to this,
other than sometimes a one extra vmexit after migration.

Otherwise there is always a risk of the following scenario:

1. We migrate with nested_run_pending=0 (but don't restore all the state
yet, like that HV_X64_MSR_VP_ASSIST_PAGE msr,
or just the guest memory map is not up to date, guest is in smm or something
like that)

2. Userspace calls some ioctl that causes a nested vmexit

This can happen today if the userspace calls
kvm_arch_vcpu_ioctl_get_mpstate -> kvm_apic_accept_events -> kvm_check_nested_events

3. Userspace finally sets correct guest's msrs, correct guest memory map and only
then calls KVM_RUN

This means that at (2) we can't map and write the evmcs/vmcs12/vmcb12 even
but we have to do so to complete the nested vmexit.

To some extent, the entry to the nested mode after a migration is only complete
when we process the KVM_REQ_GET_NESTED_STATE_PAGES, so we shoudn't interrupt it.

This will allow us to avoid dealing with KVM_REQ_GET_NESTED_STATE_PAGES on
nested vmexit path at all.

Best regards,
Maxim Levitsky