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Based on kernel version 2.6.39.1. Page generated on 2011-06-03 13:47 EST.

1	The x86 kvm shadow mmu
2	======================
3	
4	The mmu (in arch/x86/kvm, files mmu.[ch] and paging_tmpl.h) is responsible
5	for presenting a standard x86 mmu to the guest, while translating guest
6	physical addresses to host physical addresses.
7	
8	The mmu code attempts to satisfy the following requirements:
9	
10	- correctness: the guest should not be able to determine that it is running
11	               on an emulated mmu except for timing (we attempt to comply
12	               with the specification, not emulate the characteristics of
13	               a particular implementation such as tlb size)
14	- security:    the guest must not be able to touch host memory not assigned
15	               to it
16	- performance: minimize the performance penalty imposed by the mmu
17	- scaling:     need to scale to large memory and large vcpu guests
18	- hardware:    support the full range of x86 virtualization hardware
19	- integration: Linux memory management code must be in control of guest memory
20	               so that swapping, page migration, page merging, transparent
21	               hugepages, and similar features work without change
22	- dirty tracking: report writes to guest memory to enable live migration
23	               and framebuffer-based displays
24	- footprint:   keep the amount of pinned kernel memory low (most memory
25	               should be shrinkable)
26	- reliability:  avoid multipage or GFP_ATOMIC allocations
27	
28	Acronyms
29	========
30	
31	pfn   host page frame number
32	hpa   host physical address
33	hva   host virtual address
34	gfn   guest frame number
35	gpa   guest physical address
36	gva   guest virtual address
37	ngpa  nested guest physical address
38	ngva  nested guest virtual address
39	pte   page table entry (used also to refer generically to paging structure
40	      entries)
41	gpte  guest pte (referring to gfns)
42	spte  shadow pte (referring to pfns)
43	tdp   two dimensional paging (vendor neutral term for NPT and EPT)
44	
45	Virtual and real hardware supported
46	===================================
47	
48	The mmu supports first-generation mmu hardware, which allows an atomic switch
49	of the current paging mode and cr3 during guest entry, as well as
50	two-dimensional paging (AMD's NPT and Intel's EPT).  The emulated hardware
51	it exposes is the traditional 2/3/4 level x86 mmu, with support for global
52	pages, pae, pse, pse36, cr0.wp, and 1GB pages.  Work is in progress to support
53	exposing NPT capable hardware on NPT capable hosts.
54	
55	Translation
56	===========
57	
58	The primary job of the mmu is to program the processor's mmu to translate
59	addresses for the guest.  Different translations are required at different
60	times:
61	
62	- when guest paging is disabled, we translate guest physical addresses to
63	  host physical addresses (gpa->hpa)
64	- when guest paging is enabled, we translate guest virtual addresses, to
65	  guest physical addresses, to host physical addresses (gva->gpa->hpa)
66	- when the guest launches a guest of its own, we translate nested guest
67	  virtual addresses, to nested guest physical addresses, to guest physical
68	  addresses, to host physical addresses (ngva->ngpa->gpa->hpa)
69	
70	The primary challenge is to encode between 1 and 3 translations into hardware
71	that support only 1 (traditional) and 2 (tdp) translations.  When the
72	number of required translations matches the hardware, the mmu operates in
73	direct mode; otherwise it operates in shadow mode (see below).
74	
75	Memory
76	======
77	
78	Guest memory (gpa) is part of the user address space of the process that is
79	using kvm.  Userspace defines the translation between guest addresses and user
80	addresses (gpa->hva); note that two gpas may alias to the same hva, but not
81	vice versa.
82	
83	These hvas may be backed using any method available to the host: anonymous
84	memory, file backed memory, and device memory.  Memory might be paged by the
85	host at any time.
86	
87	Events
88	======
89	
90	The mmu is driven by events, some from the guest, some from the host.
91	
92	Guest generated events:
93	- writes to control registers (especially cr3)
94	- invlpg/invlpga instruction execution
95	- access to missing or protected translations
96	
97	Host generated events:
98	- changes in the gpa->hpa translation (either through gpa->hva changes or
99	  through hva->hpa changes)
100	- memory pressure (the shrinker)
101	
102	Shadow pages
103	============
104	
105	The principal data structure is the shadow page, 'struct kvm_mmu_page'.  A
106	shadow page contains 512 sptes, which can be either leaf or nonleaf sptes.  A
107	shadow page may contain a mix of leaf and nonleaf sptes.
108	
109	A nonleaf spte allows the hardware mmu to reach the leaf pages and
110	is not related to a translation directly.  It points to other shadow pages.
111	
112	A leaf spte corresponds to either one or two translations encoded into
113	one paging structure entry.  These are always the lowest level of the
114	translation stack, with optional higher level translations left to NPT/EPT.
115	Leaf ptes point at guest pages.
116	
117	The following table shows translations encoded by leaf ptes, with higher-level
118	translations in parentheses:
119	
120	 Non-nested guests:
121	  nonpaging:     gpa->hpa
122	  paging:        gva->gpa->hpa
123	  paging, tdp:   (gva->)gpa->hpa
124	 Nested guests:
125	  non-tdp:       ngva->gpa->hpa  (*)
126	  tdp:           (ngva->)ngpa->gpa->hpa
127	
128	(*) the guest hypervisor will encode the ngva->gpa translation into its page
129	    tables if npt is not present
130	
131	Shadow pages contain the following information:
132	  role.level:
133	    The level in the shadow paging hierarchy that this shadow page belongs to.
134	    1=4k sptes, 2=2M sptes, 3=1G sptes, etc.
135	  role.direct:
136	    If set, leaf sptes reachable from this page are for a linear range.
137	    Examples include real mode translation, large guest pages backed by small
138	    host pages, and gpa->hpa translations when NPT or EPT is active.
139	    The linear range starts at (gfn << PAGE_SHIFT) and its size is determined
140	    by role.level (2MB for first level, 1GB for second level, 0.5TB for third
141	    level, 256TB for fourth level)
142	    If clear, this page corresponds to a guest page table denoted by the gfn
143	    field.
144	  role.quadrant:
145	    When role.cr4_pae=0, the guest uses 32-bit gptes while the host uses 64-bit
146	    sptes.  That means a guest page table contains more ptes than the host,
147	    so multiple shadow pages are needed to shadow one guest page.
148	    For first-level shadow pages, role.quadrant can be 0 or 1 and denotes the
149	    first or second 512-gpte block in the guest page table.  For second-level
150	    page tables, each 32-bit gpte is converted to two 64-bit sptes
151	    (since each first-level guest page is shadowed by two first-level
152	    shadow pages) so role.quadrant takes values in the range 0..3.  Each
153	    quadrant maps 1GB virtual address space.
154	  role.access:
155	    Inherited guest access permissions in the form uwx.  Note execute
156	    permission is positive, not negative.
157	  role.invalid:
158	    The page is invalid and should not be used.  It is a root page that is
159	    currently pinned (by a cpu hardware register pointing to it); once it is
160	    unpinned it will be destroyed.
161	  role.cr4_pae:
162	    Contains the value of cr4.pae for which the page is valid (e.g. whether
163	    32-bit or 64-bit gptes are in use).
164	  role.nxe:
165	    Contains the value of efer.nxe for which the page is valid.
166	  role.cr0_wp:
167	    Contains the value of cr0.wp for which the page is valid.
168	  gfn:
169	    Either the guest page table containing the translations shadowed by this
170	    page, or the base page frame for linear translations.  See role.direct.
171	  spt:
172	    A pageful of 64-bit sptes containing the translations for this page.
173	    Accessed by both kvm and hardware.
174	    The page pointed to by spt will have its page->private pointing back
175	    at the shadow page structure.
176	    sptes in spt point either at guest pages, or at lower-level shadow pages.
177	    Specifically, if sp1 and sp2 are shadow pages, then sp1->spt[n] may point
178	    at __pa(sp2->spt).  sp2 will point back at sp1 through parent_pte.
179	    The spt array forms a DAG structure with the shadow page as a node, and
180	    guest pages as leaves.
181	  gfns:
182	    An array of 512 guest frame numbers, one for each present pte.  Used to
183	    perform a reverse map from a pte to a gfn. When role.direct is set, any
184	    element of this array can be calculated from the gfn field when used, in
185	    this case, the array of gfns is not allocated. See role.direct and gfn.
186	  slot_bitmap:
187	    A bitmap containing one bit per memory slot.  If the page contains a pte
188	    mapping a page from memory slot n, then bit n of slot_bitmap will be set
189	    (if a page is aliased among several slots, then it is not guaranteed that
190	    all slots will be marked).
191	    Used during dirty logging to avoid scanning a shadow page if none if its
192	    pages need tracking.
193	  root_count:
194	    A counter keeping track of how many hardware registers (guest cr3 or
195	    pdptrs) are now pointing at the page.  While this counter is nonzero, the
196	    page cannot be destroyed.  See role.invalid.
197	  multimapped:
198	    Whether there exist multiple sptes pointing at this page.
199	  parent_pte/parent_ptes:
200	    If multimapped is zero, parent_pte points at the single spte that points at
201	    this page's spt.  Otherwise, parent_ptes points at a data structure
202	    with a list of parent_ptes.
203	  unsync:
204	    If true, then the translations in this page may not match the guest's
205	    translation.  This is equivalent to the state of the tlb when a pte is
206	    changed but before the tlb entry is flushed.  Accordingly, unsync ptes
207	    are synchronized when the guest executes invlpg or flushes its tlb by
208	    other means.  Valid for leaf pages.
209	  unsync_children:
210	    How many sptes in the page point at pages that are unsync (or have
211	    unsynchronized children).
212	  unsync_child_bitmap:
213	    A bitmap indicating which sptes in spt point (directly or indirectly) at
214	    pages that may be unsynchronized.  Used to quickly locate all unsychronized
215	    pages reachable from a given page.
216	
217	Reverse map
218	===========
219	
220	The mmu maintains a reverse mapping whereby all ptes mapping a page can be
221	reached given its gfn.  This is used, for example, when swapping out a page.
222	
223	Synchronized and unsynchronized pages
224	=====================================
225	
226	The guest uses two events to synchronize its tlb and page tables: tlb flushes
227	and page invalidations (invlpg).
228	
229	A tlb flush means that we need to synchronize all sptes reachable from the
230	guest's cr3.  This is expensive, so we keep all guest page tables write
231	protected, and synchronize sptes to gptes when a gpte is written.
232	
233	A special case is when a guest page table is reachable from the current
234	guest cr3.  In this case, the guest is obliged to issue an invlpg instruction
235	before using the translation.  We take advantage of that by removing write
236	protection from the guest page, and allowing the guest to modify it freely.
237	We synchronize modified gptes when the guest invokes invlpg.  This reduces
238	the amount of emulation we have to do when the guest modifies multiple gptes,
239	or when the a guest page is no longer used as a page table and is used for
240	random guest data.
241	
242	As a side effect we have to resynchronize all reachable unsynchronized shadow
243	pages on a tlb flush.
244	
245	
246	Reaction to events
247	==================
248	
249	- guest page fault (or npt page fault, or ept violation)
250	
251	This is the most complicated event.  The cause of a page fault can be:
252	
253	  - a true guest fault (the guest translation won't allow the access) (*)
254	  - access to a missing translation
255	  - access to a protected translation
256	    - when logging dirty pages, memory is write protected
257	    - synchronized shadow pages are write protected (*)
258	  - access to untranslatable memory (mmio)
259	
260	  (*) not applicable in direct mode
261	
262	Handling a page fault is performed as follows:
263	
264	 - if needed, walk the guest page tables to determine the guest translation
265	   (gva->gpa or ngpa->gpa)
266	   - if permissions are insufficient, reflect the fault back to the guest
267	 - determine the host page
268	   - if this is an mmio request, there is no host page; call the emulator
269	     to emulate the instruction instead
270	 - walk the shadow page table to find the spte for the translation,
271	   instantiating missing intermediate page tables as necessary
272	 - try to unsynchronize the page
273	   - if successful, we can let the guest continue and modify the gpte
274	 - emulate the instruction
275	   - if failed, unshadow the page and let the guest continue
276	 - update any translations that were modified by the instruction
277	
278	invlpg handling:
279	
280	  - walk the shadow page hierarchy and drop affected translations
281	  - try to reinstantiate the indicated translation in the hope that the
282	    guest will use it in the near future
283	
284	Guest control register updates:
285	
286	- mov to cr3
287	  - look up new shadow roots
288	  - synchronize newly reachable shadow pages
289	
290	- mov to cr0/cr4/efer
291	  - set up mmu context for new paging mode
292	  - look up new shadow roots
293	  - synchronize newly reachable shadow pages
294	
295	Host translation updates:
296	
297	  - mmu notifier called with updated hva
298	  - look up affected sptes through reverse map
299	  - drop (or update) translations
300	
301	Emulating cr0.wp
302	================
303	
304	If tdp is not enabled, the host must keep cr0.wp=1 so page write protection
305	works for the guest kernel, not guest guest userspace.  When the guest
306	cr0.wp=1, this does not present a problem.  However when the guest cr0.wp=0,
307	we cannot map the permissions for gpte.u=1, gpte.w=0 to any spte (the
308	semantics require allowing any guest kernel access plus user read access).
309	
310	We handle this by mapping the permissions to two possible sptes, depending
311	on fault type:
312	
313	- kernel write fault: spte.u=0, spte.w=1 (allows full kernel access,
314	  disallows user access)
315	- read fault: spte.u=1, spte.w=0 (allows full read access, disallows kernel
316	  write access)
317	
318	(user write faults generate a #PF)
319	
320	Large pages
321	===========
322	
323	The mmu supports all combinations of large and small guest and host pages.
324	Supported page sizes include 4k, 2M, 4M, and 1G.  4M pages are treated as
325	two separate 2M pages, on both guest and host, since the mmu always uses PAE
326	paging.
327	
328	To instantiate a large spte, four constraints must be satisfied:
329	
330	- the spte must point to a large host page
331	- the guest pte must be a large pte of at least equivalent size (if tdp is
332	  enabled, there is no guest pte and this condition is satisified)
333	- if the spte will be writeable, the large page frame may not overlap any
334	  write-protected pages
335	- the guest page must be wholly contained by a single memory slot
336	
337	To check the last two conditions, the mmu maintains a ->write_count set of
338	arrays for each memory slot and large page size.  Every write protected page
339	causes its write_count to be incremented, thus preventing instantiation of
340	a large spte.  The frames at the end of an unaligned memory slot have
341	artificically inflated ->write_counts so they can never be instantiated.
342	
343	Further reading
344	===============
345	
346	- NPT presentation from KVM Forum 2008
347	  http://www.linux-kvm.org/wiki/images/c/c8/KvmForum2008%24kdf2008_21.pdf
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