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

Sean Christopherson
2022-11-17T20:05:04Z

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?

> 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.

> 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.