[PATCH v4 0/9] KVM: x86: Apply NX mitigation more precisely

Sean Christopherson
2022-08-30T23:55:45Z

Note, this applies on Yosry's stats series (there's a trivial-but-subtle
conflict in the TDP MMU shadow page accounting).
https://lore.kernel.org/all/20220823004639.2387269-1-yosryahmed@google.com

Precisely track (via kvm_mmu_page) if a non-huge page is being forced
and use that info to avoid unnecessarily forcing smaller page sizes in
disallowed_hugepage_adjust().

KVM incorrectly assumes that the NX huge page mitigation is the only
scenario where KVM will create a non-leaf page instead of a huge page.
As a result, if the original source of huge page incompatibility goes
away, the NX mitigation is enabled, and KVM encounters an present shadow
page when attempting to install a huge page, KVM will force a smaller page
regardless of whether or not a smaller page is actually necessary to
satisfy the NX huge page mitigation.

Unnecessarily forcing small pages can result in degraded guest performance,
especially on larger VMs. The bug was originally discovered when testing
dirty log performance, as KVM would leave small pages lying around when
zapping collapsible SPTEs. That case was indadvertantly fixed by commit
5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty
logging"), but other scenarios are still affected, e.g. KVM will not
rebuild a huge page if the mmu_notifier zaps a range of PTEs because the
primary MMU is creating a huge page.

v4:
- Collect reviews. [Mingwei]
- Add comment to document possible_nx_huge_pages. [Mingwei]
- Drop extra memory barriers. [Paolo]
- Document ordering providing by TDP SPTE helpers. [Paolo]

v3:
- https://lore.kernel.org/all/20220805230513.148869-1-seanjc@google.com
- Bug the VM if KVM attempts to double account a shadow page that
disallows a NX huge page. [David]
- Split the rename to separate patch. [Paolo]
- Rename more NX huge page variables/functions. [David]
- Combine and tweak the comments about enforcing the NX huge page
mitigation for non-paging MMUs. [Paolo, David]
- Call out that the shadow MMU holds mmu_lock for write and doesn't need
to manual handle memory ordering when accounting NX huge pages. [David]
- Add a smp_rmb() when unlinking shadow pages in the TDP MMU.
- Rename spte_to_sp() to spte_to_child_sp(). [David]
- Collect reviews. [David]
- Tweak the changelog for the final patch to call out that precise
accounting addresses real world performance bugs. [Paolo]
- Reword the changelog for the patch to (almost) always tag disallowed
NX huge pages, and call out that it doesn't fix the TDP MMU. [David]

v2: Rebase, tweak a changelog accordingly.

v1: https://lore.kernel.org/all/20220409003847.819686-1-seanjc@google.com

Mingwei Zhang (1):
KVM: x86/mmu: explicitly check nx_hugepage in
disallowed_hugepage_adjust()

Sean Christopherson (8):
KVM: x86/mmu: Bug the VM if KVM attempts to double count an NX huge
page
KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked
KVM: x86/mmu: Rename NX huge pages fields/functions for consistency
KVM: x86/mmu: Properly account NX huge page workaround for nonpaging
MMUs
KVM: x86/mmu: Document implicit barriers/ordering in TDP MMU shared
mode
KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting
SPTE
KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual
pages
KVM: x86/mmu: Add helper to convert SPTE value to its shadow page

arch/x86/include/asm/kvm_host.h | 30 +++++----
arch/x86/kvm/mmu/mmu.c | 115 ++++++++++++++++++++------------
arch/x86/kvm/mmu/mmu_internal.h | 33 ++++-----
arch/x86/kvm/mmu/paging_tmpl.h | 6 +-
arch/x86/kvm/mmu/spte.c | 12 ++++
arch/x86/kvm/mmu/spte.h | 17 +++++
arch/x86/kvm/mmu/tdp_iter.h | 6 ++
arch/x86/kvm/mmu/tdp_mmu.c | 48 +++++++------
arch/x86/kvm/mmu/tdp_mmu.h | 2 +
9 files changed, 176 insertions(+), 93 deletions(-)

base-commit: 6f87cacaf4fe95288ac4cbe01a2fadc5d2b8b936
--
2.37.2.672.g94769d06f0-goog