On Mon, May 08, 2023, Mathias Krause wrote:
This is a backport of the CR0.WP KVM series[1] to Linux v5.15. It differs from the v6.1 backport as in needing additional prerequisite patches from Lai Jiangshan (and fixes for those) to ensure the assumption it's safe to let CR0.WP be a guest owned bit still stand.
NAK.
The CR0.WP changes also very subtly rely on commit 2ba676774dfc ("KVM: x86/mmu: cleanup computation of MMU roles for two-dimensional paging"), which hardcodes WP=1 in the mmu role. Without that, KVM will end up in a weird state when reinitializing the MMU context without reloading the root, as KVM will effectively change the role of an active root. E.g. child pages in the legacy MMU will have a mix of WP=0 and WP=1 in their role.
The inconsistency may or may not cause functional problems (I honestly don't know), but this missed dependency is exactly the type of problem that I am/was worried about with respect to backporting these changes all the way to 5.15. I'm simply not comfortable backporting these changes due to the number of modifications and enhancements that we've made to the TDP MMU, and to KVM's MMU handling in general, between 5.15 and 6.1.