On 2024/7/23 13:59, Richard Henderson wrote:
On 7/23/24 15:29, LIU Zhiwei wrote:
The more detailed information about its meaning is in priviledged
1.13 specification. More exactly, in 3.6.4. Misaligned Atomicity
Granule PMA.
The specification said:
"The misaligned atomicity granule PMA applies only to AMOs, loads and
stores defined in the base
ISAs, and loads and stores of no more than MXLEN bits defined in the
F, D, and Q extensions. For an
instruction in that set, if all accessed bytes lie within the same
misaligned atomicity granule, the
instruction will not raise an exception for reasons of address
alignment, and the instruction will give
rise to only one memory operation for the purposes of RVWMO—i.e., it
will execute atomically."
That's the reason why I do not apply zama16b to compressed instructions.
Given the non-specificity of this paragraph, I think not specifically
calling out compressed forms of the base ISA is simply a documentation
error. In general, the compressed ISA is supposed to be a smaller
encoding of the exact same instruction as the standard ISA.
I will confirm this with the RISC-V community. Thanks.
However! It does explicitly say "no more than MXLEN bits", which
means that an RV32/RV64 check is appropriate for FLD/FSD, since MXLEN
may be less than 64.
Yes. That's true. Although I don't know why MXLEN is needed as F, D or
Q don't depend on MXLEN. We can implement D extension on RV32 CPU.
In addition, your change for AMOs is incomplete. From the text:
If a misaligned AMO accesses a region that does not specify a
misaligned
atomicity granule PMA, or if not all accessed bytes lie within the same
misaligned atomicity granule, then an exception is raised.
The second clause corresponds exactly with the Arm FEAT_LSE2.
See check_lse2_align in target/arm/tcg/translate-a64.c.
r~
PS: The first clause is similar to Arm access to pages marked as
Device memory, for which all misaligned accesses trap. I didn't dig
deep enough to see how PMAs are defined to suggest how that might be
applied.
It's more complex than I once thought. I will do more work before
sending next version.
Thanks,
Zhiwei
