x86/emul: Simplfy emulation state setup

The current code to set up emulation state is ad-hoc and error prone.

 * Consistently zero all emulation state structures.
 * Avoid explicitly initialising some state to 0.
 * Explicitly identify all input and output state in x86_emulate_ctxt.  This
   involves rearanging some fields.
 * Have x86_decode() explicitly initalise all output state at its start.

While making the above changes, two minor tweaks:

 * Move the calculation of hvmemul_ctxt->ctxt.swint_emulate from
   _hvm_emulate_one() to hvm_emulate_init_once().  It doesn't need
   recalculating for each instruction.
 * Change force_writeback to being a boolean, to match its use.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>

xen/arch/x86/hvm/emulate.c

@@ -1770,13 +1770,6 @@ static int _hvm_emulate_one(struct hvm_emulate_ctxt *hvmemul_ctxt,
 
     vio->mmio_retry = 0;
 
-    if ( cpu_has_vmx )
-        hvmemul_ctxt->ctxt.swint_emulate = x86_swint_emulate_none;
-    else if ( cpu_has_svm_nrips )
-        hvmemul_ctxt->ctxt.swint_emulate = x86_swint_emulate_icebp;
-    else
-        hvmemul_ctxt->ctxt.swint_emulate = x86_swint_emulate_all;
-
     rc = x86_emulate(&hvmemul_ctxt->ctxt, ops);
 
     if ( rc == X86EMUL_OKAY && vio->mmio_retry )
@@ -1947,14 +1940,23 @@ void hvm_emulate_init_once(
     struct hvm_emulate_ctxt *hvmemul_ctxt,
     struct cpu_user_regs *regs)
 {
-    hvmemul_ctxt->intr_shadow = hvm_funcs.get_interrupt_shadow(current);
-    hvmemul_ctxt->ctxt.regs = regs;
-    hvmemul_ctxt->ctxt.force_writeback = 1;
-    hvmemul_ctxt->seg_reg_accessed = 0;
-    hvmemul_ctxt->seg_reg_dirty = 0;
-    hvmemul_ctxt->set_context = 0;
+    struct vcpu *curr = current;
+
+    memset(hvmemul_ctxt, 0, sizeof(*hvmemul_ctxt));
+
+    hvmemul_ctxt->intr_shadow = hvm_funcs.get_interrupt_shadow(curr);
     hvmemul_get_seg_reg(x86_seg_cs, hvmemul_ctxt);
     hvmemul_get_seg_reg(x86_seg_ss, hvmemul_ctxt);
+
+    hvmemul_ctxt->ctxt.regs = regs;
+    hvmemul_ctxt->ctxt.force_writeback = true;
+
+    if ( cpu_has_vmx )
+        hvmemul_ctxt->ctxt.swint_emulate = x86_swint_emulate_none;
+    else if ( cpu_has_svm_nrips )
+        hvmemul_ctxt->ctxt.swint_emulate = x86_swint_emulate_icebp;
+    else
+        hvmemul_ctxt->ctxt.swint_emulate = x86_swint_emulate_all;
 }
 
 void hvm_emulate_init_per_insn(

xen/arch/x86/mm.c

@@ -5337,7 +5337,14 @@ int ptwr_do_page_fault(struct vcpu *v, unsigned long addr,
     struct domain *d = v->domain;
     struct page_info *page;
     l1_pgentry_t      pte;
-    struct ptwr_emulate_ctxt ptwr_ctxt;
+    struct ptwr_emulate_ctxt ptwr_ctxt = {
+        .ctxt = {
+            .regs = regs,
+            .addr_size = is_pv_32bit_domain(d) ? 32 : BITS_PER_LONG,
+            .sp_size   = is_pv_32bit_domain(d) ? 32 : BITS_PER_LONG,
+            .swint_emulate = x86_swint_emulate_none,
+        },
+    };
     int rc;
 
     /* Attempt to read the PTE that maps the VA being accessed. */
@@ -5363,11 +5370,6 @@ int ptwr_do_page_fault(struct vcpu *v, unsigned long addr,
         goto bail;
     }
 
-    ptwr_ctxt.ctxt.regs = regs;
-    ptwr_ctxt.ctxt.force_writeback = 0;
-    ptwr_ctxt.ctxt.addr_size = ptwr_ctxt.ctxt.sp_size =
-        is_pv_32bit_domain(d) ? 32 : BITS_PER_LONG;
-    ptwr_ctxt.ctxt.swint_emulate = x86_swint_emulate_none;
     ptwr_ctxt.cr2 = addr;
     ptwr_ctxt.pte = pte;
 

xen/arch/x86/mm/shadow/common.c

@@ -385,8 +385,9 @@ const struct x86_emulate_ops *shadow_init_emulation(
     struct vcpu *v = current;
     unsigned long addr;
 
+    memset(sh_ctxt, 0, sizeof(*sh_ctxt));
+
     sh_ctxt->ctxt.regs = regs;
-    sh_ctxt->ctxt.force_writeback = 0;
     sh_ctxt->ctxt.swint_emulate = x86_swint_emulate_none;
 
     if ( is_pv_vcpu(v) )
@@ -396,7 +397,6 @@ const struct x86_emulate_ops *shadow_init_emulation(
     }
 
     /* Segment cache initialisation. Primed with CS. */
-    sh_ctxt->valid_seg_regs = 0;
     creg = hvm_get_seg_reg(x86_seg_cs, sh_ctxt);
 
     /* Work out the emulation mode. */

xen/arch/x86/x86_emulate/x86_emulate.c

@@ -1904,6 +1904,7 @@ x86_decode(
     state->regs = ctxt->regs;
     state->eip = ctxt->regs->eip;
 
+    /* Initialise output state in x86_emulate_ctxt */
     ctxt->retire.byte = 0;
 
     op_bytes = def_op_bytes = ad_bytes = def_ad_bytes = ctxt->addr_size/8;

xen/arch/x86/x86_emulate/x86_emulate.h

@@ -410,6 +410,23 @@ struct cpu_user_regs;
 
 struct x86_emulate_ctxt
 {
+    /*
+     * Input-only state:
+     */
+
+    /* Software event injection support. */
+    enum x86_swint_emulation swint_emulate;
+
+    /* Set this if writes may have side effects. */
+    bool force_writeback;
+
+    /* Caller data that can be used by x86_emulate_ops' routines. */
+    void *data;
+
+    /*
+     * Input/output state:
+     */
+
     /* Register state before/after emulation. */
     struct cpu_user_regs *regs;
 
@@ -419,15 +436,13 @@ struct x86_emulate_ctxt
     /* Stack pointer width in bits (16, 32 or 64). */
     unsigned int sp_size;
 
-    /* Canonical opcode (see below). */
+    /*
+     * Output-only state:
+     */
+
+    /* Canonical opcode (see below) (valid only on X86EMUL_OKAY). */
     unsigned int opcode;
 
-    /* Software event injection support. */
-    enum x86_swint_emulation swint_emulate;
-
-    /* Set this if writes may have side effects. */
-    uint8_t force_writeback;
-
     /* Retirement state, set by the emulator (valid only on X86EMUL_OKAY). */
     union {
         struct {
@@ -437,9 +452,6 @@ struct x86_emulate_ctxt
         } flags;
         uint8_t byte;
     } retire;
-
-    /* Caller data that can be used by x86_emulate_ops' routines. */
-    void *data;
 };
 
 /*