target/arm: Postpone interpretation of stage 2 descriptor attribute bits

In the original Arm v8 two-stage translation, both stage 1 and stage
2 specify memory attributes (memory type, cacheability,
shareability); these are then combined to produce the overall memory
attributes for the whole stage 1+2 access.  In QEMU we implement this
by having get_phys_addr() fill in an ARMCacheAttrs struct, and we
convert both the stage 1 and stage 2 attribute bit formats to the
same encoding (an 8-bit attribute value matching the MAIR_EL1 fields,
plus a 2-bit shareability value).

The new FEAT_S2FWB feature allows the guest to enable a different
interpretation of the attribute bits in the stage 2 descriptors.
These bits can now be used to control details of how the stage 1 and
2 attributes should be combined (for instance they can say "always
use the stage 1 attributes" or "ignore the stage 1 attributes and
always be Device memory").  This means we need to pass the raw bit
information for stage 2 down to the function which combines the stage
1 and stage 2 information.

Add a field to ARMCacheAttrs that indicates whether the attrs field
should be interpreted as MAIR format, or as the raw stage 2 attribute
bits from the descriptor, and store the appropriate values when
filling in cacheattrs.

We only need to interpret the attrs field in a few places:
 * in do_ats_write(), where we know to expect a MAIR value
   (there is no ATS instruction to do a stage-2-only walk)
 * in S1_ptw_translate(), where we want to know whether the
   combined S1 + S2 attributes indicate Device memory that
   should provoke a fault
 * in combine_cacheattrs(), which does the S1 + S2 combining
Update those places accordingly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220505183950.2781801-2-peter.maydell@linaro.org

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