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+Support SIGILL to the guest
+Currently FEX will just close an application if it encounters a block with an invalid instruction.

+We can support giving the guest a SIGILL.

+This should be done in two steps to reduce the amount of immediate work

+

+1) Support the use-case of SIGILL + guest handler terminate

+  - This means that once the frontend encounters and invalid operation.

+  - Save Thread state

+  - End the Block

+  - Call a SIGILL host side handler

+  - Pass the signal information to the guest

+  - Invoke the guests signal handler with the known good information

+  - Guest will invoke terminate, thus taking down the application and FEX with it

+ 2) Once step 1 is done, support use cases where an application is testing for instructions and safely recovering

+  - Guest will modify the passed in ucontext once it has caught the sigill

+  - Most likely modifying RIP to jump past the instruction, or using longjump to escape

+  - Will likely also modify another GPR to know the failure case

+  - Once the guest has returned from its signal, we need to read the state back in to our context and safely continue

+  - This can benefit more than SIGILL later once we have some form of state recovery for the other signals.
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