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

[Nick Gasson]

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.

Yes the current implementation uses the frame pointer for signing/authentication so `-XX:UseBranchProtection=pac-ret` turns on `PreserveFramePointer`. 

For now I can make another patch that disables PAC-RET when `Arguments::enable_preview()` is true.

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