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docs: kvm: Convert mmu.txt to ReST format
- Use document title and chapter markups; - Add markups for tables; - Add markups for literal blocks; - Add blank lines and adjust indentation. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Mauro Carvalho Chehab
Paolo Bonzini
parent
75e7fcdb4a
commit
037d1f92ef
@ -13,6 +13,7 @@ KVM
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halt-polling
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hypercalls
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locking
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mmu
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msr
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vcpu-requests
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@ -1,3 +1,6 @@
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.. SPDX-License-Identifier: GPL-2.0
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======================
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The x86 kvm shadow mmu
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======================
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@ -7,27 +10,37 @@ physical addresses to host physical addresses.
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The mmu code attempts to satisfy the following requirements:
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- correctness: the guest should not be able to determine that it is running
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- correctness:
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the guest should not be able to determine that it is running
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on an emulated mmu except for timing (we attempt to comply
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with the specification, not emulate the characteristics of
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a particular implementation such as tlb size)
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- security: the guest must not be able to touch host memory not assigned
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- security:
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the guest must not be able to touch host memory not assigned
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to it
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- performance: minimize the performance penalty imposed by the mmu
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- scaling: need to scale to large memory and large vcpu guests
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- hardware: support the full range of x86 virtualization hardware
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- integration: Linux memory management code must be in control of guest memory
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- performance:
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minimize the performance penalty imposed by the mmu
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- scaling:
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need to scale to large memory and large vcpu guests
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- hardware:
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support the full range of x86 virtualization hardware
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- integration:
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Linux memory management code must be in control of guest memory
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so that swapping, page migration, page merging, transparent
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hugepages, and similar features work without change
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- dirty tracking: report writes to guest memory to enable live migration
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- dirty tracking:
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report writes to guest memory to enable live migration
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and framebuffer-based displays
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- footprint: keep the amount of pinned kernel memory low (most memory
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- footprint:
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keep the amount of pinned kernel memory low (most memory
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should be shrinkable)
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- reliability: avoid multipage or GFP_ATOMIC allocations
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- reliability:
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avoid multipage or GFP_ATOMIC allocations
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Acronyms
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========
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==== ====================================================================
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pfn host page frame number
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hpa host physical address
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hva host virtual address
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@ -41,6 +54,7 @@ pte page table entry (used also to refer generically to paging structure
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gpte guest pte (referring to gfns)
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spte shadow pte (referring to pfns)
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tdp two dimensional paging (vendor neutral term for NPT and EPT)
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==== ====================================================================
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Virtual and real hardware supported
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===================================
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@ -90,11 +104,13 @@ Events
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The mmu is driven by events, some from the guest, some from the host.
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Guest generated events:
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- writes to control registers (especially cr3)
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- invlpg/invlpga instruction execution
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- access to missing or protected translations
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Host generated events:
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- changes in the gpa->hpa translation (either through gpa->hva changes or
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through hva->hpa changes)
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- memory pressure (the shrinker)
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@ -117,16 +133,19 @@ Leaf ptes point at guest pages.
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The following table shows translations encoded by leaf ptes, with higher-level
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translations in parentheses:
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Non-nested guests:
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Non-nested guests::
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nonpaging: gpa->hpa
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paging: gva->gpa->hpa
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paging, tdp: (gva->)gpa->hpa
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Nested guests:
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Nested guests::
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non-tdp: ngva->gpa->hpa (*)
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tdp: (ngva->)ngpa->gpa->hpa
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(*) the guest hypervisor will encode the ngva->gpa translation into its page
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tables if npt is not present
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(*) the guest hypervisor will encode the ngva->gpa translation into its page
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tables if npt is not present
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Shadow pages contain the following information:
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role.level:
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@ -291,28 +310,41 @@ Handling a page fault is performed as follows:
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- if the RSV bit of the error code is set, the page fault is caused by guest
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accessing MMIO and cached MMIO information is available.
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- walk shadow page table
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- check for valid generation number in the spte (see "Fast invalidation of
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MMIO sptes" below)
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- cache the information to vcpu->arch.mmio_gva, vcpu->arch.mmio_access and
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vcpu->arch.mmio_gfn, and call the emulator
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- If both P bit and R/W bit of error code are set, this could possibly
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be handled as a "fast page fault" (fixed without taking the MMU lock). See
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the description in Documentation/virt/kvm/locking.txt.
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- if needed, walk the guest page tables to determine the guest translation
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(gva->gpa or ngpa->gpa)
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- if permissions are insufficient, reflect the fault back to the guest
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- determine the host page
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- if this is an mmio request, there is no host page; cache the info to
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vcpu->arch.mmio_gva, vcpu->arch.mmio_access and vcpu->arch.mmio_gfn
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- walk the shadow page table to find the spte for the translation,
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instantiating missing intermediate page tables as necessary
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- If this is an mmio request, cache the mmio info to the spte and set some
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reserved bit on the spte (see callers of kvm_mmu_set_mmio_spte_mask)
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- try to unsynchronize the page
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- if successful, we can let the guest continue and modify the gpte
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- emulate the instruction
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- if failed, unshadow the page and let the guest continue
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- update any translations that were modified by the instruction
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invlpg handling:
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@ -324,10 +356,12 @@ invlpg handling:
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Guest control register updates:
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- mov to cr3
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- look up new shadow roots
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- synchronize newly reachable shadow pages
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- mov to cr0/cr4/efer
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- set up mmu context for new paging mode
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- look up new shadow roots
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- synchronize newly reachable shadow pages
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@ -358,6 +392,7 @@ on fault type:
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(user write faults generate a #PF)
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In the first case there are two additional complications:
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- if CR4.SMEP is enabled: since we've turned the page into a kernel page,
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the kernel may now execute it. We handle this by also setting spte.nx.
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If we get a user fetch or read fault, we'll change spte.u=1 and
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@ -446,4 +481,3 @@ Further reading
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- NPT presentation from KVM Forum 2008
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http://www.linux-kvm.org/images/c/c8/KvmForum2008%24kdf2008_21.pdf
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