From ada51a9de7375ef8933fcaa1af9cb61a7fe0ceef Mon Sep 17 00:00:00 2001 From: David Matlack Date: Wed, 22 Jun 2022 15:27:09 -0400 Subject: KVM: x86/mmu: Extend Eager Page Splitting to nested MMUs Add support for Eager Page Splitting pages that are mapped by nested MMUs. Walk through the rmap first splitting all 1GiB pages to 2MiB pages, and then splitting all 2MiB pages to 4KiB pages. Note, Eager Page Splitting is limited to nested MMUs as a policy rather than due to any technical reason (the sp->role.guest_mode check could just be deleted and Eager Page Splitting would work correctly for all shadow MMU pages). There is really no reason to support Eager Page Splitting for tdp_mmu=N, since such support will eventually be phased out, and there is no current use case supporting Eager Page Splitting on hosts where TDP is either disabled or unavailable in hardware. Furthermore, future improvements to nested MMU scalability may diverge the code from the legacy shadow paging implementation. These improvements will be simpler to make if Eager Page Splitting does not have to worry about legacy shadow paging. Splitting huge pages mapped by nested MMUs requires dealing with some extra complexity beyond that of the TDP MMU: (1) The shadow MMU has a limit on the number of shadow pages that are allowed to be allocated. So, as a policy, Eager Page Splitting refuses to split if there are KVM_MIN_FREE_MMU_PAGES or fewer pages available. (2) Splitting a huge page may end up re-using an existing lower level shadow page tables. This is unlike the TDP MMU which always allocates new shadow page tables when splitting. (3) When installing the lower level SPTEs, they must be added to the rmap which may require allocating additional pte_list_desc structs. Case (2) is especially interesting since it may require a TLB flush, unlike the TDP MMU which can fully split huge pages without any TLB flushes. Specifically, an existing lower level page table may point to even lower level page tables that are not fully populated, effectively unmapping a portion of the huge page, which requires a flush. As of this commit, a flush is always done always after dropping the huge page and before installing the lower level page table. This TLB flush could instead be delayed until the MMU lock is about to be dropped, which would batch flushes for multiple splits. However these flushes should be rare in practice (a huge page must be aliased in multiple SPTEs and have been split for NX Huge Pages in only some of them). Flushing immediately is simpler to plumb and also reduces the chances of tripping over a CPU bug (e.g. see iTLB multihit). [ This commit is based off of the original implementation of Eager Page Splitting from Peter in Google's kernel from 2016. ] Suggested-by: Peter Feiner Signed-off-by: David Matlack Message-Id: <20220516232138.1783324-23-dmatlack@google.com> Signed-off-by: Paolo Bonzini --- Documentation/admin-guide/kernel-parameters.txt | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) (limited to 'Documentation/admin-guide') diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 97c16aa2f53f..329f0f274e2b 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2418,8 +2418,7 @@ the KVM_CLEAR_DIRTY ioctl, and only for the pages being cleared. - Eager page splitting currently only supports splitting - huge pages mapped by the TDP MMU. + Eager page splitting is only supported when kvm.tdp_mmu=Y. Default is Y (on). -- cgit v1.2.3