Re: [PATCH 02/13] KVM: nSVM: don't call nested_sync_control_from_vmcb02 on each VM exit

Maxim Levitsky
2022-11-21T11:13:28Z

On Thu, 2022-11-17 at 20:04 +0000, Sean Christopherson wrote:
> On Thu, Nov 17, 2022, Maxim Levitsky wrote:
> > Calling nested_sync_control_from_vmcb02 on each VM exit (nested or not),
> > was an attempt to keep the int_ctl field in the vmcb12 cache
> > up to date on each VM exit.
>
> This doesn't mesh with the reasoning in commit 2d8a42be0e2b ("KVM: nSVM: synchronize
> VMCB controls updated by the processor on every vmexit"), which states that the
> goal is to keep svm->nested.ctl.* synchronized, not vmcb12.  Or is nested.ctl the
> cache you are referring to?

Thanks for digging that commit out.

My reasoning was that cache contains both control and 'save' fields, and
we don't update 'save' fields on each VM exit.

For control it indeed looks like int_ctl and eventinj are the only fields
that are updated by the CPU, although IMHO they don't *need* to be updated
until we do a nested VM exit, because the VM isn't supposed to look at vmcb while it
is in use by the CPU, its state is undefined.

For migration though, this does look like a problem. It can be fixed during
reading the nested state but it is a hack as well.

My idea was as you had seen in the patches it to unify int_ctl handling,
since some bits might need to be copied to vmcb12 but some to vmcb01,
and we happened to have none of these so far, and it "happened" to work.

Do you have an idea on how to do this cleanly? I can just leave this as is
and only sync the bits of int_ctl from vmcb02 to vmcb01 on nested VM exit.
Ugly but would work.

>
> > However all other fields in the vmcb12 cache are not kept up to  date,
>
> IIUC, this isn't technically true.  They are up-to-date because they're never
> modified by hardware.

In both save and control cache. In control cache indeed looks like the
fields are kept up to date.

Best regards,
Maxim Levitsky

>
> > therefore for consistency it is better to do this on a nested VM exit only.
>
> Again, IIUC, this actually introduces an inconsistency because it leaves stale
> state in svm->nested.ctl, whereas the existing code ensures all state in
> svm->nested.ctl is fresh immediately after non-nested VM-Exit.
>