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.
8 The mmu code attempts to satisfy the following requirements:
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
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
26 - reliability: avoid multipage or GFP_ATOMIC allocations
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
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)
45 Virtual and real hardware supported
46 ===================================
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.
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
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)
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).
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
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
90 The mmu is driven by events, some from the guest, some from the host.
92 Guest generated events:
93 - writes to control registers (especially cr3)
94 - invlpg/invlpga instruction execution
95 - access to missing or protected translations
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)
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.
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.
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.
117 The following table shows translations encoded by leaf ptes, with higher-level
118 translations in parentheses:
122 paging: gva->gpa->hpa
123 paging, tdp: (gva->)gpa->hpa
125 non-tdp: ngva->gpa->hpa (*)
126 tdp: (ngva->)ngpa->gpa->hpa
128 (*) the guest hypervisor will encode the ngva->gpa translation into its page
129 tables if npt is not present
131 Shadow pages contain the following information:
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.
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
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.
155 Inherited guest access permissions in the form uwx. Note execute
156 permission is positive, not negative.
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.
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).
165 Contains the value of efer.nxe for which the page is valid.
167 Contains the value of cr0.wp for which the page is valid.
169 Contains the value of cr4.smep && !cr0.wp for which the page is valid
170 (pages for which this is true are different from other pages; see the
171 treatment of cr0.wp=0 below).
173 Contains the value of cr4.smap && !cr0.wp for which the page is valid
174 (pages for which this is true are different from other pages; see the
175 treatment of cr0.wp=0 below).
177 Is 1 if the page is valid in system management mode. This field
178 determines which of the kvm_memslots array was used to build this
179 shadow page; it is also used to go back from a struct kvm_mmu_page
180 to a memslot, through the kvm_memslots_for_spte_role macro and
183 Either the guest page table containing the translations shadowed by this
184 page, or the base page frame for linear translations. See role.direct.
186 A pageful of 64-bit sptes containing the translations for this page.
187 Accessed by both kvm and hardware.
188 The page pointed to by spt will have its page->private pointing back
189 at the shadow page structure.
190 sptes in spt point either at guest pages, or at lower-level shadow pages.
191 Specifically, if sp1 and sp2 are shadow pages, then sp1->spt[n] may point
192 at __pa(sp2->spt). sp2 will point back at sp1 through parent_pte.
193 The spt array forms a DAG structure with the shadow page as a node, and
194 guest pages as leaves.
196 An array of 512 guest frame numbers, one for each present pte. Used to
197 perform a reverse map from a pte to a gfn. When role.direct is set, any
198 element of this array can be calculated from the gfn field when used, in
199 this case, the array of gfns is not allocated. See role.direct and gfn.
201 A counter keeping track of how many hardware registers (guest cr3 or
202 pdptrs) are now pointing at the page. While this counter is nonzero, the
203 page cannot be destroyed. See role.invalid.
205 The reverse mapping for the pte/ptes pointing at this page's spt. If
206 parent_ptes bit 0 is zero, only one spte points at this pages and
207 parent_ptes points at this single spte, otherwise, there exists multiple
208 sptes pointing at this page and (parent_ptes & ~0x1) points at a data
209 structure with a list of parent_ptes.
211 If true, then the translations in this page may not match the guest's
212 translation. This is equivalent to the state of the tlb when a pte is
213 changed but before the tlb entry is flushed. Accordingly, unsync ptes
214 are synchronized when the guest executes invlpg or flushes its tlb by
215 other means. Valid for leaf pages.
217 How many sptes in the page point at pages that are unsync (or have
218 unsynchronized children).
220 A bitmap indicating which sptes in spt point (directly or indirectly) at
221 pages that may be unsynchronized. Used to quickly locate all unsychronized
222 pages reachable from a given page.
224 Generation number of the page. It is compared with kvm->arch.mmu_valid_gen
225 during hash table lookup, and used to skip invalidated shadow pages (see
226 "Zapping all pages" below.)
228 Only present on 32-bit hosts, where a 64-bit spte cannot be written
229 atomically. The reader uses this while running out of the MMU lock
230 to detect in-progress updates and retry them until the writer has
232 write_flooding_count:
233 A guest may write to a page table many times, causing a lot of
234 emulations if the page needs to be write-protected (see "Synchronized
235 and unsynchronized pages" below). Leaf pages can be unsynchronized
236 so that they do not trigger frequent emulation, but this is not
237 possible for non-leafs. This field counts the number of emulations
238 since the last time the page table was actually used; if emulation
239 is triggered too frequently on this page, KVM will unmap the page
240 to avoid emulation in the future.
245 The mmu maintains a reverse mapping whereby all ptes mapping a page can be
246 reached given its gfn. This is used, for example, when swapping out a page.
248 Synchronized and unsynchronized pages
249 =====================================
251 The guest uses two events to synchronize its tlb and page tables: tlb flushes
252 and page invalidations (invlpg).
254 A tlb flush means that we need to synchronize all sptes reachable from the
255 guest's cr3. This is expensive, so we keep all guest page tables write
256 protected, and synchronize sptes to gptes when a gpte is written.
258 A special case is when a guest page table is reachable from the current
259 guest cr3. In this case, the guest is obliged to issue an invlpg instruction
260 before using the translation. We take advantage of that by removing write
261 protection from the guest page, and allowing the guest to modify it freely.
262 We synchronize modified gptes when the guest invokes invlpg. This reduces
263 the amount of emulation we have to do when the guest modifies multiple gptes,
264 or when the a guest page is no longer used as a page table and is used for
267 As a side effect we have to resynchronize all reachable unsynchronized shadow
268 pages on a tlb flush.
274 - guest page fault (or npt page fault, or ept violation)
276 This is the most complicated event. The cause of a page fault can be:
278 - a true guest fault (the guest translation won't allow the access) (*)
279 - access to a missing translation
280 - access to a protected translation
281 - when logging dirty pages, memory is write protected
282 - synchronized shadow pages are write protected (*)
283 - access to untranslatable memory (mmio)
285 (*) not applicable in direct mode
287 Handling a page fault is performed as follows:
289 - if the RSV bit of the error code is set, the page fault is caused by guest
290 accessing MMIO and cached MMIO information is available.
291 - walk shadow page table
292 - check for valid generation number in the spte (see "Fast invalidation of
294 - cache the information to vcpu->arch.mmio_gva, vcpu->arch.access and
295 vcpu->arch.mmio_gfn, and call the emulator
296 - If both P bit and R/W bit of error code are set, this could possibly
297 be handled as a "fast page fault" (fixed without taking the MMU lock). See
298 the description in Documentation/virtual/kvm/locking.txt.
299 - if needed, walk the guest page tables to determine the guest translation
300 (gva->gpa or ngpa->gpa)
301 - if permissions are insufficient, reflect the fault back to the guest
302 - determine the host page
303 - if this is an mmio request, there is no host page; cache the info to
304 vcpu->arch.mmio_gva, vcpu->arch.access and vcpu->arch.mmio_gfn
305 - walk the shadow page table to find the spte for the translation,
306 instantiating missing intermediate page tables as necessary
307 - If this is an mmio request, cache the mmio info to the spte and set some
308 reserved bit on the spte (see callers of kvm_mmu_set_mmio_spte_mask)
309 - try to unsynchronize the page
310 - if successful, we can let the guest continue and modify the gpte
311 - emulate the instruction
312 - if failed, unshadow the page and let the guest continue
313 - update any translations that were modified by the instruction
317 - walk the shadow page hierarchy and drop affected translations
318 - try to reinstantiate the indicated translation in the hope that the
319 guest will use it in the near future
321 Guest control register updates:
324 - look up new shadow roots
325 - synchronize newly reachable shadow pages
327 - mov to cr0/cr4/efer
328 - set up mmu context for new paging mode
329 - look up new shadow roots
330 - synchronize newly reachable shadow pages
332 Host translation updates:
334 - mmu notifier called with updated hva
335 - look up affected sptes through reverse map
336 - drop (or update) translations
341 If tdp is not enabled, the host must keep cr0.wp=1 so page write protection
342 works for the guest kernel, not guest guest userspace. When the guest
343 cr0.wp=1, this does not present a problem. However when the guest cr0.wp=0,
344 we cannot map the permissions for gpte.u=1, gpte.w=0 to any spte (the
345 semantics require allowing any guest kernel access plus user read access).
347 We handle this by mapping the permissions to two possible sptes, depending
350 - kernel write fault: spte.u=0, spte.w=1 (allows full kernel access,
351 disallows user access)
352 - read fault: spte.u=1, spte.w=0 (allows full read access, disallows kernel
355 (user write faults generate a #PF)
357 In the first case there are two additional complications:
358 - if CR4.SMEP is enabled: since we've turned the page into a kernel page,
359 the kernel may now execute it. We handle this by also setting spte.nx.
360 If we get a user fetch or read fault, we'll change spte.u=1 and
361 spte.nx=gpte.nx back. For this to work, KVM forces EFER.NX to 1 when
362 shadow paging is in use.
363 - if CR4.SMAP is disabled: since the page has been changed to a kernel
364 page, it can not be reused when CR4.SMAP is enabled. We set
365 CR4.SMAP && !CR0.WP into shadow page's role to avoid this case. Note,
366 here we do not care the case that CR4.SMAP is enabled since KVM will
367 directly inject #PF to guest due to failed permission check.
369 To prevent an spte that was converted into a kernel page with cr0.wp=0
370 from being written by the kernel after cr0.wp has changed to 1, we make
371 the value of cr0.wp part of the page role. This means that an spte created
372 with one value of cr0.wp cannot be used when cr0.wp has a different value -
373 it will simply be missed by the shadow page lookup code. A similar issue
374 exists when an spte created with cr0.wp=0 and cr4.smep=0 is used after
375 changing cr4.smep to 1. To avoid this, the value of !cr0.wp && cr4.smep
376 is also made a part of the page role.
381 The mmu supports all combinations of large and small guest and host pages.
382 Supported page sizes include 4k, 2M, 4M, and 1G. 4M pages are treated as
383 two separate 2M pages, on both guest and host, since the mmu always uses PAE
386 To instantiate a large spte, four constraints must be satisfied:
388 - the spte must point to a large host page
389 - the guest pte must be a large pte of at least equivalent size (if tdp is
390 enabled, there is no guest pte and this condition is satisfied)
391 - if the spte will be writeable, the large page frame may not overlap any
392 write-protected pages
393 - the guest page must be wholly contained by a single memory slot
395 To check the last two conditions, the mmu maintains a ->write_count set of
396 arrays for each memory slot and large page size. Every write protected page
397 causes its write_count to be incremented, thus preventing instantiation of
398 a large spte. The frames at the end of an unaligned memory slot have
399 artificially inflated ->write_counts so they can never be instantiated.
401 Zapping all pages (page generation count)
402 =========================================
404 For the large memory guests, walking and zapping all pages is really slow
405 (because there are a lot of pages), and also blocks memory accesses of
406 all VCPUs because it needs to hold the MMU lock.
408 To make it be more scalable, kvm maintains a global generation number
409 which is stored in kvm->arch.mmu_valid_gen. Every shadow page stores
410 the current global generation-number into sp->mmu_valid_gen when it
411 is created. Pages with a mismatching generation number are "obsolete".
413 When KVM need zap all shadow pages sptes, it just simply increases the global
414 generation-number then reload root shadow pages on all vcpus. As the VCPUs
415 create new shadow page tables, the old pages are not used because of the
416 mismatching generation number.
418 KVM then walks through all pages and zaps obsolete pages. While the zap
419 operation needs to take the MMU lock, the lock can be released periodically
420 so that the VCPUs can make progress.
422 Fast invalidation of MMIO sptes
423 ===============================
425 As mentioned in "Reaction to events" above, kvm will cache MMIO
426 information in leaf sptes. When a new memslot is added or an existing
427 memslot is changed, this information may become stale and needs to be
428 invalidated. This also needs to hold the MMU lock while walking all
429 shadow pages, and is made more scalable with a similar technique.
431 MMIO sptes have a few spare bits, which are used to store a
432 generation number. The global generation number is stored in
433 kvm_memslots(kvm)->generation, and increased whenever guest memory info
434 changes. This generation number is distinct from the one described in
435 the previous section.
437 When KVM finds an MMIO spte, it checks the generation number of the spte.
438 If the generation number of the spte does not equal the global generation
439 number, it will ignore the cached MMIO information and handle the page
440 fault through the slow path.
442 Since only 19 bits are used to store generation-number on mmio spte, all
443 pages are zapped when there is an overflow.
445 Unfortunately, a single memory access might access kvm_memslots(kvm) multiple
446 times, the last one happening when the generation number is retrieved and
447 stored into the MMIO spte. Thus, the MMIO spte might be created based on
448 out-of-date information, but with an up-to-date generation number.
450 To avoid this, the generation number is incremented again after synchronize_srcu
451 returns; thus, the low bit of kvm_memslots(kvm)->generation is only 1 during a
452 memslot update, while some SRCU readers might be using the old copy. We do not
453 want to use an MMIO sptes created with an odd generation number, and we can do
454 this without losing a bit in the MMIO spte. The low bit of the generation
455 is not stored in MMIO spte, and presumed zero when it is extracted out of the
456 spte. If KVM is unlucky and creates an MMIO spte while the low bit is 1,
457 the next access to the spte will always be a cache miss.
463 - NPT presentation from KVM Forum 2008
464 http://www.linux-kvm.org/wiki/images/c/c8/KvmForum2008%24kdf2008_21.pdf