x86/spec-ctrl: Issue VERW during IST exit to Xen

There is a corner case where e.g. an NMI hitting an exit-to-guest path after
SPEC_CTRL_EXIT_TO_* would have run the entire NMI handler *after* the VERW
flush to scrub potentially sensitive data from uarch buffers.

In order to compensate, issue VERW when exiting to Xen from an IST entry.

SPEC_CTRL_EXIT_TO_XEN already has two reads of spec_ctrl_flags off the stack,
and we're about to add a third.  Load the field into %ebx, and list the
register as clobbered.

%r12 has been arranged to be the ist_exit signal, so add this as an input
dependency and use it to identify when to issue a VERW.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 3ee6066bcd)

xen/arch/x86/x86_64/entry.S

@@ -671,7 +671,7 @@ UNLIKELY_START(ne, exit_cr3)
 UNLIKELY_END(exit_cr3)
 
         /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
-        SPEC_CTRL_EXIT_TO_XEN     /* Req: %r14=end, Clob: acd */
+        SPEC_CTRL_EXIT_TO_XEN     /* Req: %r12=ist_exit %r14=end, Clob: abcd */
 
         RESTORE_ALL adj=8
         iretq

xen/include/asm-x86/spec_ctrl_asm.h

@@ -362,10 +362,12 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
  */
 .macro SPEC_CTRL_EXIT_TO_XEN
 /*
- * Requires %r14=stack_end
- * Clobbers %rax, %rcx, %rdx
+ * Requires %r12=ist_exit, %r14=stack_end
+ * Clobbers %rax, %rbx, %rcx, %rdx
  */
-    testb $SCF_ist_sc_msr, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
+    movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %ebx
+
+    testb $SCF_ist_sc_msr, %bl
     jz .L\@_skip_sc_msr
 
     /*
@@ -376,7 +378,7 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
      */
     xor %edx, %edx
 
-    testb $SCF_use_shadow, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
+    testb $SCF_use_shadow, %bl
     jz .L\@_skip_sc_msr
 
     mov STACK_CPUINFO_FIELD(shadow_spec_ctrl)(%r14), %eax
@@ -385,8 +387,16 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
 
 .L\@_skip_sc_msr:
 
-    /* TODO VERW */
+    test %r12, %r12
+    jz .L\@_skip_ist_exit
 
+    /* Logically DO_SPEC_CTRL_COND_VERW but without the %rsp=cpuinfo dependency */
+    testb $SCF_verw, %bl
+    jz .L\@_skip_verw
+    verw STACK_CPUINFO_FIELD(verw_sel)(%r14)
+.L\@_skip_verw:
+
+.L\@_skip_ist_exit:
 .endm
 
 #endif /* __ASSEMBLY__ */