RFR: 8287325: AArch64: fix virtual threads with -XX:UseBranchProtection=pac-ret

[Ron Pressler]

On Tue, 7 Jun 2022 16:42:14 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

> The continuation free/thaw mechanism relies on being able to move thread stacks around in memory.  However when PAC is enabled on supported AArch64 CPUs, the saved LR on the stack contains a "pointer authentication code" signed with the stack pointer at the time the frame was created.  When a stack frame is relocated we need to re-sign the LR with the new stack pointer to ensure it authenticates successfully when the method returns.
> 
> Introduced `ContinuationHelper::return_pc_at()` to avoid directly reading the saved PC from the stack in shared code.  On AArch64 with PAC it enabled it strips the PAC from the address after reading it, on all other platforms it just loads the PC from the stack as before.

Could you explain what it is that this change attempts to do? Does _every_ frame's returns address need to be patched on both freeze and thaw? If so, the slow path always has to be taken as we don't walk frames on the fast path. That slows down virtual threads by ~5-20x.

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PR: https://git.openjdk.java.net/jdk/pull/9067