[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen staging] xen/wait: Describe RSB safety
commit da74c951e4e58080583daaad373b0d38a3f8bc83 Author: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> AuthorDate: Tue Jun 14 16:18:36 2022 +0100 Commit: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> CommitDate: Fri Aug 5 12:16:59 2022 +0100 xen/wait: Describe RSB safety It turns out that we do in fact have RSB safety here, but not for obvious reasons. Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> --- xen/common/wait.c | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/xen/common/wait.c b/xen/common/wait.c index e45345ede7..86d3b15419 100644 --- a/xen/common/wait.c +++ b/xen/common/wait.c @@ -209,6 +209,27 @@ void check_wakeup_from_wait(void) do_softirq(); } + /* + * We are about to jump into a deeper call tree. In principle, this risks + * executing more RET than CALL instructions, and underflowing the RSB. + * + * However, we are pinned to the same CPU as previously. Therefore, + * either: + * + * 1) We've scheduled another vCPU in the meantime, and the context + * switch path has (by default) issued IBPB which flushes the RSB, or + * + * 2) We're still in the same context. Returning back to the deeper + * call tree is resuming the execution path we left, and remains + * balanced as far as that logic is concerned. + * + * In fact, the path through the scheduler will execute more CALL + * than RET instructions, making the RSB unbalanced in the safe + * direction. + * + * Therefore, no actions are necessary here to maintain RSB safety. + */ + /* * Hand-rolled longjmp(). * -- generated by git-patchbot for /home/xen/git/xen.git#staging
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