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| author | Andrew Waterman <aswaterman@gmail.com> | 2018-11-30 14:01:53 -0800 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2018-11-30 14:01:53 -0800 |
| commit | 71356274220e31b42d6b62dc4f6deaeeef9f42de (patch) | |
| tree | f4aca15c2178ea56ef50d1a466f65890d6d2d912 /src/machine.tex | |
| parent | 7b6a4ed7d180caebb9d9985be5026471ac47cd9a (diff) | |
| download | riscv-isa-manual-71356274220e31b42d6b62dc4f6deaeeef9f42de.zip riscv-isa-manual-71356274220e31b42d6b62dc4f6deaeeef9f42de.tar.gz riscv-isa-manual-71356274220e31b42d6b62dc4f6deaeeef9f42de.tar.bz2 | |
Extend mstatus.TW to U-mode for M/U systems (#286)
Diffstat (limited to 'src/machine.tex')
| -rw-r--r-- | src/machine.tex | 20 |
1 files changed, 14 insertions, 6 deletions
diff --git a/src/machine.tex b/src/machine.tex index 35f2c30..e3d61bf 100644 --- a/src/machine.tex +++ b/src/machine.tex @@ -715,12 +715,14 @@ hooks necessary to lazily populate shadow page tables. \end{commentary} The TW (Timeout Wait) bit supports intercepting the WFI instruction (see -Section~\ref{wfi}). When TW=0, the WFI instruction is permitted in S-mode. -When TW=1, if WFI is executed in S-mode, and it does not complete within an -implementation-specific, bounded time limit, the WFI instruction causes an -illegal instruction trap. The time limit may always be 0, in which case WFI -always causes an illegal instruction trap in S-mode when TW=1. -TW is hard-wired to 0 when S-mode is not supported. +Section~\ref{wfi}). When TW=0, the TW mechanism does not inhibit the WFI +instruction from executing in the next-lowest privileged mode. When TW=1, +then if WFI is executed in any less-privileged mode, and it does not complete +within an implementation-specific, bounded time limit, the WFI instruction +causes an illegal instruction exception. The time limit may always be 0, in which +case WFI always causes an illegal instruction exception in less-privileged modes +when TW=1. TW is hard-wired to 0 when there are no modes less privileged than +M. \begin{commentary} Trapping the WFI @@ -728,6 +730,12 @@ instruction can trigger a world switch to another guest OS, rather than wastefully idling in the current guest. \end{commentary} +When S-mode is implemented, then executing WFI in U-mode causes an illegal +instruction exception, unless it completes within an implementation-specific, +bounded time limit. A future revision of this specification might add +a feature that allows S-mode to selectively permit WFI in U-mode. Such +a feature would only be active when TW=0. + The TSR (Trap SRET) bit supports intercepting the supervisor exception return instruction, SRET. When TSR=1, attempts to execute SRET while executing in S-mode will raise an illegal instruction exception. When TSR=0, this |