x86/svm: Improve diagnostics when svm_get_insn_len() fails

Sadly, a lone:

  (XEN) emulate.c:156:d2v0 svm_get_insn_len: Mismatch between expected and actual instruction: eip = fffff804564139c0

on the console is of no use trying to identify what went wrong.  Dump as much
state as we can to help identify what went wrong.

  (XEN) Insn mismatch: Expected opcode 0xf0031, modrm 0, got nrip_len 3, emul_len 3
  (XEN) SVM Insn len emulation failed (1): d1v0 64bit @ 0008:0010475f -> 0f 01 f9 0f 31 5b 31 ff 31 c0 e9 c2 db ff ff 00

Drop the debug-only early exit if the sources of length disagree, because the
only effect it has it to avoid the more detailed analysis of what went wrong.

Reported-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Brian Woods <brian.woods@amd.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

xen/arch/x86/hvm/svm/emulate.c

@@ -64,7 +64,6 @@ static unsigned long svm_nextrip_insn_length(struct vcpu *v)
  */
 unsigned int svm_get_insn_len(struct vcpu *v, unsigned int instr_enc)
 {
-    struct vmcb_struct *vmcb = v->arch.hvm.svm.vmcb;
     struct hvm_emulate_ctxt ctxt;
     struct x86_emulate_state *state;
     unsigned long nrip_len, emul_len;
@@ -92,15 +91,6 @@ unsigned int svm_get_insn_len(struct vcpu *v, unsigned int instr_enc)
     modrm_mod = x86_insn_modrm(state, &modrm_rm, &modrm_reg);
     x86_emulate_free_state(state);
 
-#ifndef NDEBUG
-    if ( nrip_len && nrip_len != emul_len )
-    {
-        gprintk(XENLOG_WARNING, "insn-len[%02x]=%lu (exp %lu)\n",
-                ctxt.ctxt.opcode, nrip_len, emul_len);
-        return nrip_len;
-    }
-#endif
-
     /* Extract components from instr_enc. */
     instr_modrm  = instr_enc & 0xff;
     instr_opcode = instr_enc >> 8;
@@ -116,9 +106,12 @@ unsigned int svm_get_insn_len(struct vcpu *v, unsigned int instr_enc)
             return emul_len;
     }
 
-    gdprintk(XENLOG_WARNING,
-             "%s: Mismatch between expected and actual instruction: "
-             "eip = %lx\n",  __func__, (unsigned long)vmcb->rip);
+    printk(XENLOG_G_WARNING
+           "Insn mismatch: Expected opcode %#x, modrm %#x, got nrip_len %lu, emul_len %lu\n",
+           instr_opcode, instr_modrm, nrip_len, emul_len);
+    hvm_dump_emulation_state(XENLOG_G_WARNING, "SVM Insn len",
+                             &ctxt, X86EMUL_UNHANDLEABLE);
+
     hvm_inject_hw_exception(TRAP_gp_fault, 0);
     return 0;
 }