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215 lines
7.6 KiB
215 lines
7.6 KiB
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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On x86, vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock.
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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 i.e. the SPTE_SPECIAL_MASK is set. That means we need to
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restore the saved R/X bits. This is described in more detail later below.
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2. Write-Protection: The SPTE is present and the fault is
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caused by write-protect. That means we just need to change the W bit of the
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spte.
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What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and
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SPTE_MMU_WRITEABLE bit on the spte:
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- SPTE_HOST_WRITEABLE means the gfn is writable on host.
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- SPTE_MMU_WRITEABLE means the gfn is writable on mmu. The bit is set when
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the gfn is writable on guest mmu and it is not write-protected by shadow
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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.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, or
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restore the saved R/X bits if VMX_EPT_TRACK_ACCESS mask is set, or both. This
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is safe because whenever 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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At the beginning:
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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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VCPU 0 VCPU0
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on fast page fault path:
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old_spte = *spte;
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pfn1 is swapped out:
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spte = 0;
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pfn1 is re-alloced for gfn2.
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gpte is changed to point to
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gfn2 by the guest:
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spte = pfn1;
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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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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, before we do cmpxchg, we call gfn_to_pfn_atomic()
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to pin gfn to pfn, because after gfn_to_pfn_atomic():
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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 that means it can not 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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Currently, to simplify the whole things, we disable fast page fault for
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indirect shadow page.
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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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At the beginning:
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spte.W = 0
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spte.Accessed = 1
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VCPU 0 VCPU0
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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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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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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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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, when the KVM MMU notifier is called to track accesses to a
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page (via kvm_mmu_notifier_clear_flush_young), it marks the PTE as not-present
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by clearing the RWX bits in the PTE and storing the original R & X bits in
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some unused/ignored bits. In addition, the SPTE_SPECIAL_MASK is also set on the
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PTE (using the ignored bit 62). When the VM tries to access the page later on,
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a fault is generated and the fast page fault mechanism described above is used
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to atomically restore the PTE to a Present state. The W bit is not saved when
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the PTE is marked for access tracking and during restoration to the Present
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state, the W bit is set depending on whether or not it was a write access. If
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it wasn't, then the W bit will remain clear until a write access happens, at
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which 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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