tree f5906be97197a7a625c35509a002ffe317d027ad
parent 0bfcef81e3fed72809411f084a088618bf322d78
author Nick Maniscalco <maniscalco@google.com> 1536948547 -0700
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1537456316 +0000

[kernel][arm64][mp][timer] Make arch_spinloop_pause a no-op on arm64

This change removes arch_spinloop_signal and changes arm64's
arch_spinloop_pause to issue YIELD rather than WFE.

The purpose of this change is to eliminate potential bugs that may
arise from WFE with no corresponding SEV.

In two places (timer.cpp and mp.cpp) arch_spinloop_pause, in
conjunction with arch_spinloop_signal, is used to create a kind of
condition variable, suspending execution until another CPU calls
arch_spinloop_signal.

Elsewhere (like some UART drivers), it is simply used as a hinting
no-op in busy loops.

On x64, arch_spinloop_pause is PAUSE and arch_spinloop_signal is
empty.

On arm64, arch_spinloop_pause is WFE (Wait For Event) and
arch_spinloop_signal is SEV (Send Event). WFE suspends execution until
an event is signaled (via SEV, global monitor transition, etc.). This
means that any use of WFE without a corresponding SEV (or other
mechanism like Load-Exclusive) could potentially suspend the CPU for
an indefinite period of time.

Test: On VIM2 and Eve, ran the following:
- k ut sync_ipi_tests
- k ut timer
- k timer_stress 60

ZX-2562 #comment followup

Change-Id: If2b8facef4845865d5bfe7a4d0089cd5aef791a6