1KVM Lock Overview
2=================
3
41. Acquisition Orders
5---------------------
6
7(to be written)
8
92: Exception
10------------
11
12Fast page fault:
13
14Fast page fault is the fast path which fixes the guest page fault out of
15the mmu-lock on x86. Currently, the page fault can be fast only if the
16shadow page table is present and it is caused by write-protect, that means
17we just need change the W bit of the spte.
18
19What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and
20SPTE_MMU_WRITEABLE bit on the spte:
21- SPTE_HOST_WRITEABLE means the gfn is writable on host.
22- SPTE_MMU_WRITEABLE means the gfn is writable on mmu. The bit is set when
23  the gfn is writable on guest mmu and it is not write-protected by shadow
24  page write-protection.
25
26On fast page fault path, we will use cmpxchg to atomically set the spte W
27bit if spte.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, this
28is safe because whenever changing these bits can be detected by cmpxchg.
29
30But we need carefully check these cases:
311): The mapping from gfn to pfn
32The mapping from gfn to pfn may be changed since we can only ensure the pfn
33is not changed during cmpxchg. This is a ABA problem, for example, below case
34will happen:
35
36At the beginning:
37gpte = gfn1
38gfn1 is mapped to pfn1 on host
39spte is the shadow page table entry corresponding with gpte and
40spte = pfn1
41
42   VCPU 0                           VCPU0
43on fast page fault path:
44
45   old_spte = *spte;
46                                 pfn1 is swapped out:
47                                    spte = 0;
48
49                                 pfn1 is re-alloced for gfn2.
50
51                                 gpte is changed to point to
52                                 gfn2 by the guest:
53                                    spte = pfn1;
54
55   if (cmpxchg(spte, old_spte, old_spte+W)
56	mark_page_dirty(vcpu->kvm, gfn1)
57             OOPS!!!
58
59We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
60
61For direct sp, we can easily avoid it since the spte of direct sp is fixed
62to gfn. For indirect sp, before we do cmpxchg, we call gfn_to_pfn_atomic()
63to pin gfn to pfn, because after gfn_to_pfn_atomic():
64- We have held the refcount of pfn that means the pfn can not be freed and
65  be reused for another gfn.
66- The pfn is writable that means it can not be shared between different gfns
67  by KSM.
68
69Then, we can ensure the dirty bitmaps is correctly set for a gfn.
70
71Currently, to simplify the whole things, we disable fast page fault for
72indirect shadow page.
73
742): Dirty bit tracking
75In the origin code, the spte can be fast updated (non-atomically) if the
76spte is read-only and the Accessed bit has already been set since the
77Accessed bit and Dirty bit can not be lost.
78
79But it is not true after fast page fault since the spte can be marked
80writable between reading spte and updating spte. Like below case:
81
82At the beginning:
83spte.W = 0
84spte.Accessed = 1
85
86   VCPU 0                                       VCPU0
87In mmu_spte_clear_track_bits():
88
89   old_spte = *spte;
90
91   /* 'if' condition is satisfied. */
92   if (old_spte.Accssed == 1 &&
93        old_spte.W == 0)
94      spte = 0ull;
95                                         on fast page fault path:
96                                             spte.W = 1
97                                         memory write on the spte:
98                                             spte.Dirty = 1
99
100
101   else
102      old_spte = xchg(spte, 0ull)
103
104
105   if (old_spte.Accssed == 1)
106      kvm_set_pfn_accessed(spte.pfn);
107   if (old_spte.Dirty == 1)
108      kvm_set_pfn_dirty(spte.pfn);
109      OOPS!!!
110
111The Dirty bit is lost in this case.
112
113In order to avoid this kind of issue, we always treat the spte as "volatile"
114if it can be updated out of mmu-lock, see spte_has_volatile_bits(), it means,
115the spte is always atomically updated in this case.
116
1173): flush tlbs due to spte updated
118If the spte is updated from writable to readonly, we should flush all TLBs,
119otherwise rmap_write_protect will find a read-only spte, even though the
120writable spte might be cached on a CPU's TLB.
121
122As mentioned before, the spte can be updated to writable out of mmu-lock on
123fast page fault path, in order to easily audit the path, we see if TLBs need
124be flushed caused by this reason in mmu_spte_update() since this is a common
125function to update spte (present -> present).
126
127Since the spte is "volatile" if it can be updated out of mmu-lock, we always
128atomically update the spte, the race caused by fast page fault can be avoided,
129See the comments in spte_has_volatile_bits() and mmu_spte_update().
130
1313. Reference
132------------
133
134Name:		kvm_lock
135Type:		spinlock_t
136Arch:		any
137Protects:	- vm_list
138
139Name:		kvm_count_lock
140Type:		raw_spinlock_t
141Arch:		any
142Protects:	- hardware virtualization enable/disable
143Comment:	'raw' because hardware enabling/disabling must be atomic /wrt
144		migration.
145
146Name:		kvm_arch::tsc_write_lock
147Type:		raw_spinlock
148Arch:		x86
149Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
150		- tsc offset in vmcb
151Comment:	'raw' because updating the tsc offsets must not be preempted.
152
153Name:		kvm->mmu_lock
154Type:		spinlock_t
155Arch:		any
156Protects:	-shadow page/shadow tlb entry
157Comment:	it is a spinlock since it is used in mmu notifier.
158
159Name:		kvm->srcu
160Type:		srcu lock
161Arch:		any
162Protects:	- kvm->memslots
163		- kvm->buses
164Comment:	The srcu read lock must be held while accessing memslots (e.g.
165		when using gfn_to_* functions) and while accessing in-kernel
166		MMIO/PIO address->device structure mapping (kvm->buses).
167		The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
168		if it is needed by multiple functions.
169