CVE-2026-64284: KVM: x86: Ensure vendor's exit handler runs before fastpath userspace exits

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Ensure vendor's exit handler runs before fastpath userspace exits

Move the handling of fastpath userspace exits into vendor code to ensure KVM runs vendor specific operations that need to run before userspace gains control of the vCPU. E.g. for VMX (and soon to be for SVM as well), KVM needs to flush the PML buffer prior to exiting to userspace, otherwise any memory written by the final KVMRUN might never be flagged as dirty.

Note, waiting to snapshot CR0 and CR3 until svmhandleexit() is flawed in general, as that risks consuming stale state in a fastpath handler. That will be addressed in a future change.