The Arm SMMUv3 includes an optional feature equivalent to the CPU
FEAT_BBM, which permits an OS to switch a range of memory between
"covered by a huge page" and "covered by a sequence of normal pages"
without having to engage in the traditional 'break-before-make'
dance. (This is particularly important for the SMMU, because devices
performing I/O through an SMMU are less likely to be able to cope with
the window in the sequence where an access results in a translation
fault.)  The SMMU spec explicitly notes that one of the valid ways to
be a BBM level 2 compliant implementation is:
 * if there are multiple entries in the TLB for an address,
   choose one of them and use it, ignoring the others

Our SMMU TLB implementation (unlike our CPU TLB) does allow multiple
TLB entries for an address, because the translation table level is
part of the SMMUIOTLBKey, and so our IOTLB hashtable can include
entries for the same address where the leaf was at different levels
(i.e. both hugepage and normal page). Our TLB lookup implementation in
smmu_iotlb_lookup() will always find the entry with the lowest level
(i.e. it prefers the hugepage over the normal page) and ignore any
others. TLB invalidation correctly removes all TLB entries matching
the specified address or address range (unless the guest specifies the
leaf level explicitly, in which case it gets what it asked for). So we
can validly advertise support for BBML level 2.

Note that we still can't yet advertise ourselves as an SMMU v3.2,
because v3.2 requires support for the S2FWB feature, which we don't
yet implement.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 20220426160422.2353158-4-peter.maydell@linaro.org