[Xen-devel] [PATCH 0/6] x86/HVM: implement memory read caching

["Jan Beulich" <JBeulich@xxxxxxxx>]

Emulation requiring device model assistance uses a form of instruction
re-execution, assuming that the second (and any further) pass takes
exactly the same path. This is a valid assumption as far use of CPU
registers goes (as those can't change without any other instruction
executing in between), but is wrong for memory accesses. In particular
it has been observed that Windows might page out buffers underneath an
instruction currently under emulation (hitting between two passes). If
the first pass translated a linear address successfully, any subsequent
pass needs to do so too, yielding the exact same translation.

Introduce a cache (used just by guest page table accesses for now) to
make sure above described assumption holds. This is a very simplistic
implementation for now: Only exact matches are satisfied (no overlaps or
partial reads or anything).

There's also some seemingly unrelated cleanup here which was found
desirable on the way.

1: x86/mm: add optional cache to GLA->GFN translation
2: x86/mm: use optional cache in guest_walk_tables()
3: x86/HVM: implement memory read caching
4: VMX: correct PDPTE load checks
5: x86/HVM: prefill cache with PDPTEs when possible
6: x86/shadow: a little bit of style cleanup

Jan



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