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| author | Andrew Waterman <andrew@sifive.com> | 2019-06-21 16:36:05 -0700 |
|---|---|---|
| committer | Andrew Waterman <andrew@sifive.com> | 2019-06-21 16:36:05 -0700 |
| commit | ec4492fe51a5186c4b0a8b83b0dac622beb341c3 (patch) | |
| tree | eb05b999264aadb2b23643fcdd7892b20a2546ca | |
| parent | 38be9a3a618f5bdfb8862b472559d477cb52e354 (diff) | |
| download | riscv-isa-manual-ec4492fe51a5186c4b0a8b83b0dac622beb341c3.zip riscv-isa-manual-ec4492fe51a5186c4b0a8b83b0dac622beb341c3.tar.gz riscv-isa-manual-ec4492fe51a5186c4b0a8b83b0dac622beb341c3.tar.bz2 | |
Clarify SBE/SFENCE interaction
| -rw-r--r-- | src/machine.tex | 13 |
1 files changed, 12 insertions, 1 deletions
diff --git a/src/machine.tex b/src/machine.tex index 6c79e4c..ca3b543 100644 --- a/src/machine.tex +++ b/src/machine.tex @@ -753,9 +753,20 @@ accesses to such pages made from S-mode with {\tt sstatus}.SUM=1. For {\em implicit} accesses to supervisor-level memory management data structures, such as page tables, endianness is always controlled by SBE. Since changing SBE alters the implementation's interpretation of these data -structures, M-mode software must follow a change to SBE by executing an +structures, if any such data structures remain in use across a change to SBE, +M-mode software must follow such a change to SBE by executing an SFENCE.VMA instruction with {\em rs1}={\tt x0} and {\em rs2}={\tt x0}. +\begin{commentary} +Only in contrived scenarios will a given memory-management data structure be +interpreted as both little-endian and big-endian. +In practice, SBE will only be changed at runtime on world switches, in which +case neither the old nor new memory-management data structure will be +reinterpreted in a different endianness. +In this case, no additional SFENCE.VMA is necessary, beyond what would +ordinarily be required for a world switch. +\end{commentary} + If S-mode is supported, an implementation may hardwire SBE so that SBE=MBE. If U-mode is supported, an implementation may hardwire UBE so that |