[Xen-devel] [PATCH v3 0/4] 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 as 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, i.e.
a form of "paging structure cache") 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: x86/HVM: prefill cache with PDPTEs when possible

As for v2, I'm omitting "VMX: correct PDPTE load checks" from v3, as I
can't currently find enough time to carry out the requested further
rework.

Jan



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