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Diffstat (limited to 'Documentation/kvm/mmu.txt')
| -rw-r--r-- | Documentation/kvm/mmu.txt | 52 |
1 files changed, 48 insertions, 4 deletions
diff --git a/Documentation/kvm/mmu.txt b/Documentation/kvm/mmu.txt index aaed6ab9d7a..142cc513665 100644 --- a/Documentation/kvm/mmu.txt +++ b/Documentation/kvm/mmu.txt @@ -77,10 +77,10 @@ Memory Guest memory (gpa) is part of the user address space of the process that is using kvm. Userspace defines the translation between guest addresses and user -addresses (gpa->hva); note that two gpas may alias to the same gva, but not +addresses (gpa->hva); note that two gpas may alias to the same hva, but not vice versa. -These gvas may be backed using any method available to the host: anonymous +These hvas may be backed using any method available to the host: anonymous memory, file backed memory, and device memory. Memory might be paged by the host at any time. @@ -161,7 +161,7 @@ Shadow pages contain the following information: role.cr4_pae: Contains the value of cr4.pae for which the page is valid (e.g. whether 32-bit or 64-bit gptes are in use). - role.cr4_nxe: + role.nxe: Contains the value of efer.nxe for which the page is valid. role.cr0_wp: Contains the value of cr0.wp for which the page is valid. @@ -180,7 +180,9 @@ Shadow pages contain the following information: guest pages as leaves. gfns: An array of 512 guest frame numbers, one for each present pte. Used to - perform a reverse map from a pte to a gfn. + perform a reverse map from a pte to a gfn. When role.direct is set, any + element of this array can be calculated from the gfn field when used, in + this case, the array of gfns is not allocated. See role.direct and gfn. slot_bitmap: A bitmap containing one bit per memory slot. If the page contains a pte mapping a page from memory slot n, then bit n of slot_bitmap will be set @@ -296,6 +298,48 @@ Host translation updates: - look up affected sptes through reverse map - drop (or update) translations +Emulating cr0.wp +================ + +If tdp is not enabled, the host must keep cr0.wp=1 so page write protection +works for the guest kernel, not guest guest userspace. When the guest +cr0.wp=1, this does not present a problem. However when the guest cr0.wp=0, +we cannot map the permissions for gpte.u=1, gpte.w=0 to any spte (the +semantics require allowing any guest kernel access plus user read access). + +We handle this by mapping the permissions to two possible sptes, depending +on fault type: + +- kernel write fault: spte.u=0, spte.w=1 (allows full kernel access, + disallows user access) +- read fault: spte.u=1, spte.w=0 (allows full read access, disallows kernel + write access) + +(user write faults generate a #PF) + +Large pages +=========== + +The mmu supports all combinations of large and small guest and host pages. +Supported page sizes include 4k, 2M, 4M, and 1G. 4M pages are treated as +two separate 2M pages, on both guest and host, since the mmu always uses PAE +paging. + +To instantiate a large spte, four constraints must be satisfied: + +- the spte must point to a large host page +- the guest pte must be a large pte of at least equivalent size (if tdp is + enabled, there is no guest pte and this condition is satisified) +- if the spte will be writeable, the large page frame may not overlap any + write-protected pages +- the guest page must be wholly contained by a single memory slot + +To check the last two conditions, the mmu maintains a ->write_count set of +arrays for each memory slot and large page size. Every write protected page +causes its write_count to be incremented, thus preventing instantiation of +a large spte. The frames at the end of an unaligned memory slot have +artificically inflated ->write_counts so they can never be instantiated. + Further reading =============== |