[PATCH 2/7] x86/emul: Fix and extend #DB trap handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Lots of this is very very broken, but we need to start somewhere.

First, the bugfix.  Hooks which use X86EMUL_DONE to skip the general emulation
still need to evaluate singlestep as part of completing the instruction.
Defer the logic until X86EMUL_DONE has been converted to X86EMUL_OKAY.

Second, the improvement.  PENDING_DBG, INTERRUPTIBILITY and ACTIVITY are
internal pipeline state which Intel exposed to software in the VMCS, and AMD
exposed a subset of in the VMCB.  Importantly, bits set in PENDING_DBG can
survive across multiple instruction boundaries if e.g. delivery of #DB is
delayed by a MovSS.

For now, introduce a full pending_dbg field into the retire union.  This keeps
the sh_page_fault() and init_context() paths working but in due course the
field will want to lose the "retire" infix.

In addition, set singlestep into pending_dbg as appropriate.  Leave the old
singlestep bitfield in place until we can adjust the callers to the new
scheme.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Jinoh Kang <jinoh.kang.kr@xxxxxxxxx>

v2:
 * Only evaluate singlestep on X86EMUL_OKAY, but do so after X86EMUL_DONE has
   been adjusted to X86EMUL_OKAY.
 * Adjust comments in light of X86EMUL_DONE not getting back to callers.
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 20 +++++++++++++-------
 xen/arch/x86/x86_emulate/x86_emulate.h | 14 +++++++++++---
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 94caec1d142c..de7f99500e3f 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -8379,13 +8379,6 @@ x86_emulate(
     if ( !mode_64bit() )
         _regs.r(ip) = (uint32_t)_regs.r(ip);
 
-    /* Should a singlestep #DB be raised? */
-    if ( rc == X86EMUL_OKAY && singlestep && !ctxt->retire.mov_ss )
-    {
-        ctxt->retire.singlestep = true;
-        ctxt->retire.sti = false;
-    }
-
     if ( rc != X86EMUL_DONE )
         *ctxt->regs = _regs;
     else
@@ -8394,6 +8387,19 @@ x86_emulate(
         rc = X86EMUL_OKAY;
     }
 
+    /* Should a singlestep #DB be raised? */
+    if ( rc == X86EMUL_OKAY && singlestep )
+    {
+        ctxt->retire.pending_dbg |= X86_DR6_BS;
+
+        /* BROKEN - TODO, merge into pending_dbg. */
+        if ( !ctxt->retire.mov_ss )
+        {
+            ctxt->retire.singlestep = true;
+            ctxt->retire.sti = false;
+        }
+    }
+
     ctxt->regs->eflags &= ~X86_EFLAGS_RF;
 
  done:
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.h 
b/xen/arch/x86/x86_emulate/x86_emulate.h
index 698750267a90..fbc023c37e34 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -588,15 +588,23 @@ struct x86_emulate_ctxt
     /* Canonical opcode (see below) (valid only on X86EMUL_OKAY). */
     unsigned int opcode;
 
-    /* Retirement state, set by the emulator (valid only on X86EMUL_OKAY). */
+    /*
+     * Retirement state, set by the emulator (valid only on X86EMUL_OKAY).
+     *
+     * TODO: all this state should be input/output from the VMCS PENDING_DBG,
+     * INTERRUPTIBILITY and ACTIVITIY fields.
+     */
     union {
-        uint8_t raw;
+        unsigned long raw;
         struct {
+            /* Accumulated %dr6 trap bits, positive polarity. */
+            unsigned int pending_dbg;
+
             bool hlt:1;          /* Instruction HLTed. */
             bool mov_ss:1;       /* Instruction sets MOV-SS irq shadow. */
             bool sti:1;          /* Instruction sets STI irq shadow. */
             bool unblock_nmi:1;  /* Instruction clears NMI blocking. */
-            bool singlestep:1;   /* Singlestepping was active. */
+            bool singlestep:1;   /* Singlestepping was active. (TODO, merge 
into pending_dbg) */
         };
     } retire;
 
-- 
2.30.2