linux-kernel - [GIT PULL] KVM: x86: MMU changes for 6.15

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Message-ID: <20250318180303.283401-3-seanjc@google.com>
Date: Tue, 18 Mar 2025 11:02:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>
Subject: [GIT PULL] KVM: x86: MMU changes for 6.15

Except for a minor cleanup, the MMU changes for 6.15 are all about adding
support for aging SPTEs without holding mmu_lock.  Details in the tag.

The following changes since commit a64dcfb451e254085a7daee5fe51bf22959d52d3:

  Linux 6.14-rc2 (2025-02-09 12:45:03 -0800)

are available in the Git repository at:

  https://github.com/kvm-x86/linux.git tags/kvm-x86-mmu-6.15

for you to fetch changes up to 0dab791f05ce2c9f0215f50cb46ed0c3126fe211:

  KVM: x86/tdp_mmu: Remove tdp_mmu_for_each_pte() (2025-02-28 09:14:20 -0800)

----------------------------------------------------------------
KVM x86/mmu changes for 6.15

Add support for "fast" aging of SPTEs in both the TDP MMU and Shadow MMU, where
"fast" means "without holding mmu_lock".  Not taking mmu_lock allows multiple
aging actions to run in parallel, and more importantly avoids stalling vCPUs,
e.g. due to holding mmu_lock for an extended duration while a vCPU is faulting
in memory.

For the TDP MMU, protect aging via RCU; the page tables are RCU-protected and
KVM doesn't need to access any metadata to age SPTEs.

For the Shadow MMU, use bit 1 of rmap pointers (bit 0 is used to terminate a
list of rmaps) to implement a per-rmap single-bit spinlock.  When aging a gfn,
acquire the rmap's spinlock with read-only permissions, which allows hardening
and optimizing the locking and aging, e.g. locking an rmap for write requires
mmu_lock to also be held.  The lock is NOT a true R/W spinlock, i.e. multiple
concurrent readers aren't supported.

To avoid forcing all SPTE updates to use atomic operations (clearing the
Accessed bit out of mmu_lock makes it inherently volatile), rework and rename
spte_has_volatile_bits() to spte_needs_atomic_update() and deliberately exclude
the Accessed bit.  KVM (and mm/) already tolerates false positives/negatives
for Accessed information, and all testing has shown that reducing the latency
of aging is far more beneficial to overall system performance than providing
"perfect" young/old information.

----------------------------------------------------------------
James Houghton (6):
      KVM: Rename kvm_handle_hva_range()
      KVM: Allow lockless walk of SPTEs when handing aging mmu_notifier event
      KVM: x86/mmu: Factor out spte atomic bit clearing routine
      KVM: x86/mmu: Don't force atomic update if only the Accessed bit is volatile
      KVM: x86/mmu: Skip shadow MMU test_young if TDP MMU reports page as young
      KVM: x86/mmu: Only check gfn age in shadow MMU if indirect_shadow_pages > 0

Nikolay Borisov (1):
      KVM: x86/tdp_mmu: Remove tdp_mmu_for_each_pte()

Sean Christopherson (6):
      KVM: x86/mmu: Always update A/D-disabled SPTEs atomically
      KVM: x86/mmu: Age TDP MMU SPTEs without holding mmu_lock
      KVM: x86/mmu: Refactor low level rmap helpers to prep for walking w/o mmu_lock
      KVM: x86/mmu: Add infrastructure to allow walking rmaps outside of mmu_lock
      KVM: x86/mmu: Add support for lockless walks of rmap SPTEs
      KVM: x86/mmu: Walk rmaps (shadow MMU) without holding mmu_lock when aging gfns

 Documentation/virt/kvm/locking.rst |   4 +-
 arch/x86/include/asm/kvm_host.h    |   4 +-
 arch/x86/kvm/Kconfig               |   1 +
 arch/x86/kvm/mmu/mmu.c             | 363 +++++++++++++++++++++++++++----------
 arch/x86/kvm/mmu/spte.c            |  31 ++--
 arch/x86/kvm/mmu/spte.h            |   2 +-
 arch/x86/kvm/mmu/tdp_iter.h        |  34 ++--
 arch/x86/kvm/mmu/tdp_mmu.c         |  45 +++--
 include/linux/kvm_host.h           |   1 +
 virt/kvm/Kconfig                   |   4 +
 virt/kvm/kvm_main.c                |  53 +++---
 11 files changed, 373 insertions(+), 169 deletions(-)