260 lines
12 KiB
ReStructuredText
260 lines
12 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=================
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KVM Lock Overview
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=================
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1. Acquisition Orders
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---------------------
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The acquisition orders for mutexes are as follows:
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- kvm->lock is taken outside vcpu->mutex
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- kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock
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- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
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them together is quite rare.
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- Unlike kvm->slots_lock, kvm->slots_arch_lock is released before
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synchronize_srcu(&kvm->srcu). Therefore kvm->slots_arch_lock
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can be taken inside a kvm->srcu read-side critical section,
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while kvm->slots_lock cannot.
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- kvm->mn_active_invalidate_count ensures that pairs of
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invalidate_range_start() and invalidate_range_end() callbacks
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use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock
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are taken on the waiting side in install_new_memslots, so MMU notifiers
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must not take either kvm->slots_lock or kvm->slots_arch_lock.
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On x86:
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- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock
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- kvm->arch.mmu_lock is an rwlock. kvm->arch.tdp_mmu_pages_lock and
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kvm->arch.mmu_unsync_pages_lock are taken inside kvm->arch.mmu_lock, and
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cannot be taken without already holding kvm->arch.mmu_lock (typically with
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``read_lock`` for the TDP MMU, thus the need for additional spinlocks).
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Everything else is a leaf: no other lock is taken inside the critical
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sections.
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2. Exception
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------------
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Fast page fault:
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Fast page fault is the fast path which fixes the guest page fault out of
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the mmu-lock on x86. Currently, the page fault can be fast in one of the
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following two cases:
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1. Access Tracking: The SPTE is not present, but it is marked for access
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tracking. That means we need to restore the saved R/X bits. This is
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described in more detail later below.
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2. Write-Protection: The SPTE is present and the fault is caused by
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write-protect. That means we just need to change the W bit of the spte.
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What we use to avoid all the race is the Host-writable bit and MMU-writable bit
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on the spte:
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- Host-writable means the gfn is writable in the host kernel page tables and in
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its KVM memslot.
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- MMU-writable means the gfn is writable in the guest's mmu and it is not
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write-protected by shadow page write-protection.
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On fast page fault path, we will use cmpxchg to atomically set the spte W
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bit if spte.HOST_WRITEABLE = 1 and spte.WRITE_PROTECT = 1, to restore the saved
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R/X bits if for an access-traced spte, or both. This is safe because whenever
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changing these bits can be detected by cmpxchg.
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But we need carefully check these cases:
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1) The mapping from gfn to pfn
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The mapping from gfn to pfn may be changed since we can only ensure the pfn
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is not changed during cmpxchg. This is a ABA problem, for example, below case
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will happen:
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+------------------------------------------------------------------------+
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| At the beginning:: |
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| |
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| gpte = gfn1 |
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| gfn1 is mapped to pfn1 on host |
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| spte is the shadow page table entry corresponding with gpte and |
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| spte = pfn1 |
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+------------------------------------------------------------------------+
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| On fast page fault path: |
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+------------------------------------+-----------------------------------+
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| CPU 0: | CPU 1: |
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+------------------------------------+-----------------------------------+
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| :: | |
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| | |
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| old_spte = *spte; | |
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+------------------------------------+-----------------------------------+
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| | pfn1 is swapped out:: |
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| | |
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| | spte = 0; |
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| | |
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| | pfn1 is re-alloced for gfn2. |
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| | |
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| | gpte is changed to point to |
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| | gfn2 by the guest:: |
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| | |
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| | spte = pfn1; |
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+------------------------------------+-----------------------------------+
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| :: |
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| if (cmpxchg(spte, old_spte, old_spte+W) |
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| mark_page_dirty(vcpu->kvm, gfn1) |
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| OOPS!!! |
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+------------------------------------------------------------------------+
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We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
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For direct sp, we can easily avoid it since the spte of direct sp is fixed
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to gfn. For indirect sp, we disabled fast page fault for simplicity.
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A solution for indirect sp could be to pin the gfn, for example via
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kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg. After the pinning:
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- We have held the refcount of pfn that means the pfn can not be freed and
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be reused for another gfn.
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- The pfn is writable and therefore it cannot be shared between different gfns
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by KSM.
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Then, we can ensure the dirty bitmaps is correctly set for a gfn.
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2) Dirty bit tracking
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In the origin code, the spte can be fast updated (non-atomically) if the
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spte is read-only and the Accessed bit has already been set since the
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Accessed bit and Dirty bit can not be lost.
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But it is not true after fast page fault since the spte can be marked
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writable between reading spte and updating spte. Like below case:
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+------------------------------------------------------------------------+
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| At the beginning:: |
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| spte.W = 0 |
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| spte.Accessed = 1 |
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+------------------------------------+-----------------------------------+
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| CPU 0: | CPU 1: |
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+------------------------------------+-----------------------------------+
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| In mmu_spte_clear_track_bits():: | |
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| old_spte = *spte; | |
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| /* 'if' condition is satisfied. */| |
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| if (old_spte.Accessed == 1 && | |
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| old_spte.W == 0) | |
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| spte = 0ull; | |
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+------------------------------------+-----------------------------------+
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| | on fast page fault path:: |
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| | spte.W = 1 |
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| | memory write on the spte:: |
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| | spte.Dirty = 1 |
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+------------------------------------+-----------------------------------+
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| :: | |
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| else | |
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| old_spte = xchg(spte, 0ull) | |
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| if (old_spte.Accessed == 1) | |
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| kvm_set_pfn_accessed(spte.pfn);| |
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| if (old_spte.Dirty == 1) | |
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| kvm_set_pfn_dirty(spte.pfn); | |
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| OOPS!!! | |
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+------------------------------------+-----------------------------------+
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The Dirty bit is lost in this case.
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In order to avoid this kind of issue, we always treat the spte as "volatile"
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if it can be updated out of mmu-lock, see spte_has_volatile_bits(), it means,
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the spte is always atomically updated in this case.
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3) flush tlbs due to spte updated
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If the spte is updated from writable to readonly, we should flush all TLBs,
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otherwise rmap_write_protect will find a read-only spte, even though the
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writable spte might be cached on a CPU's TLB.
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As mentioned before, the spte can be updated to writable out of mmu-lock on
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fast page fault path, in order to easily audit the path, we see if TLBs need
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be flushed caused by this reason in mmu_spte_update() since this is a common
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function to update spte (present -> present).
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Since the spte is "volatile" if it can be updated out of mmu-lock, we always
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atomically update the spte, the race caused by fast page fault can be avoided,
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See the comments in spte_has_volatile_bits() and mmu_spte_update().
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Lockless Access Tracking:
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This is used for Intel CPUs that are using EPT but do not support the EPT A/D
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bits. In this case, PTEs are tagged as A/D disabled (using ignored bits), and
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when the KVM MMU notifier is called to track accesses to a page (via
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kvm_mmu_notifier_clear_flush_young), it marks the PTE not-present in hardware
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by clearing the RWX bits in the PTE and storing the original R & X bits in more
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unused/ignored bits. When the VM tries to access the page later on, a fault is
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generated and the fast page fault mechanism described above is used to
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atomically restore the PTE to a Present state. The W bit is not saved when the
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PTE is marked for access tracking and during restoration to the Present state,
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the W bit is set depending on whether or not it was a write access. If it
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wasn't, then the W bit will remain clear until a write access happens, at which
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time it will be set using the Dirty tracking mechanism described above.
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3. Reference
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------------
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:Name: kvm_lock
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:Type: mutex
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:Arch: any
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:Protects: - vm_list
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:Name: kvm_count_lock
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:Type: raw_spinlock_t
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:Arch: any
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:Protects: - hardware virtualization enable/disable
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:Comment: 'raw' because hardware enabling/disabling must be atomic /wrt
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migration.
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:Name: kvm_arch::tsc_write_lock
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:Type: raw_spinlock
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:Arch: x86
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:Protects: - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
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- tsc offset in vmcb
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:Comment: 'raw' because updating the tsc offsets must not be preempted.
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:Name: kvm->mmu_lock
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:Type: spinlock_t
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:Arch: any
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:Protects: -shadow page/shadow tlb entry
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:Comment: it is a spinlock since it is used in mmu notifier.
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:Name: kvm->srcu
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:Type: srcu lock
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:Arch: any
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:Protects: - kvm->memslots
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- kvm->buses
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:Comment: The srcu read lock must be held while accessing memslots (e.g.
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when using gfn_to_* functions) and while accessing in-kernel
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MMIO/PIO address->device structure mapping (kvm->buses).
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The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
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if it is needed by multiple functions.
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:Name: blocked_vcpu_on_cpu_lock
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:Type: spinlock_t
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:Arch: x86
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:Protects: blocked_vcpu_on_cpu
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:Comment: This is a per-CPU lock and it is used for VT-d posted-interrupts.
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When VT-d posted-interrupts is supported and the VM has assigned
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devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
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protected by blocked_vcpu_on_cpu_lock, when VT-d hardware issues
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wakeup notification event since external interrupts from the
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assigned devices happens, we will find the vCPU on the list to
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wakeup.
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