x86/entry: Track the IST-ness of an entry for the exit paths

Use %r12 to hold an ist_exit boolean.  This register is zero elsewhere in the
entry/exit asm, so it only needs setting in the IST path.

As this is subtle and fragile, add check_ist_exit() to be used in debugging
builds to cross-check that the ist_exit boolean matches the entry vector.

Write check_ist_exit() it in C, because it's debug only and the logic more
complicated than I care to maintain in asm.

For now, we only need to use this signal in the exit-to-Xen path, but some
exit-to-guest paths happen in IST context too.  Check the correctness in all
exit paths to avoid the logic bit-rotting.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 21bdc25b05)

x86/entry: Partially revert IST-exit checks

The patch adding check_ist_exit() didn't account for the fact that
reset_stack_and_jump() is not an ABI-preserving boundary.  The IST-ness in
%r12 doesn't survive into the next context, and is a stale value C.

This shows up in Gitlab CI for the Clang build:

  https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/5112783827

and in OSSTest for GCC 8:

  http://logs.test-lab.xenproject.org/osstest/logs/183045/test-amd64-amd64-xl-qemuu-debianhvm-amd64/serial-pinot0.log

There's no straightforward way to reconstruct the IST-exit-ness on the
exit-to-guest path after a context switch.  For now, we only need IST-exit on
the return-to-Xen path.

Fixes: 21bdc25b05 ("x86/entry: Track the IST-ness of an entry for the exit paths")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 9b57c800b7)

xen/arch/x86/traps.c

@@ -2348,6 +2348,19 @@ void asm_domain_crash_synchronous(unsigned long addr)
         do_softirq();
 }
 
+#ifdef CONFIG_DEBUG
+void check_ist_exit(const struct cpu_user_regs *regs, bool ist_exit)
+{
+    const unsigned int ist_mask =
+        (1U << X86_EXC_NMI) | (1U << X86_EXC_DB) |
+        (1U << X86_EXC_DF)  | (1U << X86_EXC_MC);
+    uint8_t ev = regs->entry_vector;
+    bool is_ist = (ev < TRAP_nr) && ((1U << ev) & ist_mask);
+
+    ASSERT(is_ist == ist_exit);
+}
+#endif
+
 /*
  * Local variables:
  * mode: C

xen/arch/x86/x86_64/entry.S

@@ -650,8 +650,15 @@ ret_from_intr:
         .section .text.entry, "ax", @progbits
 
         ALIGN
-/* No special register assumptions. */
+/* %r12=ist_exit */
 restore_all_xen:
+
+#ifdef CONFIG_DEBUG
+        mov   %rsp, %rdi
+        mov   %r12, %rsi
+        call  check_ist_exit
+#endif
+
         /*
          * Check whether we need to switch to the per-CPU page tables, in
          * case we return to late PV exit code (from an NMI or #MC).
@@ -1032,6 +1039,10 @@ handle_ist_exception:
         INDIRECT_CALL %rdx
         mov   %r15, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         mov   %bl, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
+
+        /* This is an IST exit */
+        mov   $1, %r12d
+
         cmpb  $TRAP_nmi,UREGS_entry_vector(%rsp)
         jne   ret_from_intr