[llvm] [clang] [RISCV] Add Zicfiss support to the shadow call stack implementation. (PR #68075)

[Yeting Kuo via cfe-commits]

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@@ -57,11 +57,16 @@ compiled application or the operating system. Integrating the runtime into
 the operating system should be preferred since otherwise all thread creation
 and destruction would need to be intercepted by the application.
 
-The instrumentation makes use of the platform register ``x18`` on AArch64 and
-``x3`` (``gp``) on RISC-V. For simplicity we will refer to this as the
-``SCSReg``. On some platforms, ``SCSReg`` is reserved, and on others, it is
-designated as a scratch register.  This generally means that any code that may
-run on the same thread as code compiled with ShadowCallStack must either target
+The instrumentation makes use of the platform register ``x18`` on AArch64,
+``x3`` (``gp``) on RISC-V with software shadow stack and ``ssp`` on RISC-V with
+hardware shadow stack, which needs `Zicfiss`_ and ``-mno-forced-sw-shadow-stack``
+(default option). Note that with ``Zicfiss``_ the RISC-V backend will default to
+the hardware based shadow call stack. Users can force the RISC-V backend to
+generate the software shadow call stack with ``Zicfiss``_ by passing
+``-mforced-sw-shadow-stack``.
+For simplicity we will refer to this as the ``SCSReg``. On some platforms,
+``SCSReg`` is reserved, and on others, it is designated as a scratch register.
+This generally means that any code that may run on the same thread as code compiled with ShadowCallStack must either target
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yetingk wrote:

Done.

https://github.com/llvm/llvm-project/pull/68075