Clean up the minimums in mmu_topup_memory_caches() to document the
driving mechanisms behind the minimums.  Now that encountering an empty
cache is unlikely to trigger BUG_ON(), it is less dangerous to be more
precise when defining the minimums.

For rmaps, the logic is 1 parent PTE per level, plus a single rmap, and
prefetched rmaps.  The extra objects in the current '8 + PREFETCH'
minimum came about due to an abundance of paranoia in commit
c41ef344 ("KVM: MMU: increase per-vcpu rmap cache alloc size"),
i.e. it could have increased the minimum to 2 rmaps.  Furthermore, the
unexpected extra rmap case was killed off entirely by commits
f759e2b4 ("KVM: MMU: avoid pte_list_desc running out in
kvm_mmu_pte_write") and f5a1e9f8 ("KVM: MMU: remove call to
kvm_mmu_pte_write from walk_addr").

For the so called page cache, replace '8' with 2*PT64_ROOT_MAX_LEVEL.
The 2x multiplier is needed because the cache is used for both shadow
pages and gfn arrays for indirect MMUs.

And finally, for page headers, replace '4' with PT64_ROOT_MAX_LEVEL.

Note, KVM now supports 5-level paging, i.e. the old minimums that used a
baseline derived from 4-level paging were technically wrong.  But, KVM
always allocates roots in a separate flow, e.g. it's impossible in the
current implementation to actually need 5 new shadow pages in a single
flow.  Use PT64_ROOT_MAX_LEVEL unmodified instead of subtracting 1, as
the direct usage is likely more intuitive to uninformed readers, and the
inflated minimum is unlikely to affect functionality in practice.
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200703023545.8771-9-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>