x86: always supply .cpuid() handler to x86_emulate()

With us incremementally adding proper CPUID checks to x86_emulate()
(see commit de05bd965a ["x86emul: correct {,F}CMOV and F{,U}COMI{,P}
emulation"]) it is no longer appropriate to invoke the function with
that hook being NULL, as long as respective instructions may get used
in that case.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I've noticed the problem first with a not yet submitted patch checking
cx8 (PV 32-bit kernels occasionally use cmpxchg8b for page table
accesses), and I think we don't strictly need the change here for 4.8,
but then again it may be better to handle this properly right away.

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1542,7 +1542,7 @@ static int hvmemul_wbinvd(
     return X86EMUL_OKAY;
 }
 
-static int hvmemul_cpuid(
+int hvmemul_cpuid(
     unsigned int *eax,
     unsigned int *ebx,
     unsigned int *ecx,
@@ -1892,11 +1892,13 @@ int hvm_emulate_one_mmio(unsigned long m
         .read       = x86emul_unhandleable_rw,
         .insn_fetch = hvmemul_insn_fetch,
         .write      = mmcfg_intercept_write,
+        .cpuid      = hvmemul_cpuid,
     };
     static const struct x86_emulate_ops hvm_ro_emulate_ops_mmio = {
         .read       = x86emul_unhandleable_rw,
         .insn_fetch = hvmemul_insn_fetch,
-        .write      = mmio_ro_emulated_write
+        .write      = mmio_ro_emulated_write,
+        .cpuid      = hvmemul_cpuid,
     };
     struct mmio_ro_emulate_ctxt mmio_ro_ctxt = { .cr2 = gla };
     struct hvm_emulate_ctxt ctxt;
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -5327,6 +5327,7 @@ static const struct x86_emulate_ops ptwr
     .insn_fetch = ptwr_emulated_read,
     .write      = ptwr_emulated_write,
     .cmpxchg    = ptwr_emulated_cmpxchg,
+    .cpuid      = pv_emul_cpuid,
 };
 
 /* Write page fault handler: check if guest is trying to modify a PTE. */
@@ -5414,6 +5415,7 @@ static const struct x86_emulate_ops mmio
     .read       = x86emul_unhandleable_rw,
     .insn_fetch = ptwr_emulated_read,
     .write      = mmio_ro_emulated_write,
+    .cpuid      = pv_emul_cpuid,
 };
 
 int mmcfg_intercept_write(
@@ -5451,6 +5453,7 @@ static const struct x86_emulate_ops mmcf
     .read       = x86emul_unhandleable_rw,
     .insn_fetch = ptwr_emulated_read,
     .write      = mmcfg_intercept_write,
+    .cpuid      = pv_emul_cpuid,
 };
 
 /* Check if guest is trying to modify a r/o MMIO page. */
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -306,6 +306,7 @@ static const struct x86_emulate_ops hvm_
     .insn_fetch = hvm_emulate_insn_fetch,
     .write      = hvm_emulate_write,
     .cmpxchg    = hvm_emulate_cmpxchg,
+    .cpuid      = hvmemul_cpuid,
 };
 
 static int
@@ -374,6 +375,7 @@ static const struct x86_emulate_ops pv_s
     .insn_fetch = pv_emulate_read,
     .write      = pv_emulate_write,
     .cmpxchg    = pv_emulate_cmpxchg,
+    .cpuid      = pv_emul_cpuid,
 };
 
 const struct x86_emulate_ops *shadow_init_emulation(
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2755,6 +2755,24 @@ static int priv_op_write_msr(unsigned in
     return X86EMUL_UNHANDLEABLE;
 }
 
+int pv_emul_cpuid(unsigned int *eax, unsigned int *ebx, unsigned int *ecx,
+                  unsigned int *edx, struct x86_emulate_ctxt *ctxt)
+{
+    struct cpu_user_regs regs = *ctxt->regs;
+
+    regs._eax = *eax;
+    regs._ecx = *ecx;
+
+    pv_cpuid(&regs);
+
+    *eax = regs._eax;
+    *ebx = regs._ebx;
+    *ecx = regs._ecx;
+    *edx = regs._edx;
+
+    return X86EMUL_OKAY;
+}
+
 /* Instruction fetch with error handling. */
 #define insn_fetch(type, base, eip, limit)                                  \
 ({  unsigned long _rc, _ptr = (base) + (eip);                               \
--- a/xen/include/asm-x86/hvm/emulate.h
+++ b/xen/include/asm-x86/hvm/emulate.h
@@ -60,6 +60,12 @@ void hvm_emulate_init(
     unsigned int insn_bytes);
 void hvm_emulate_writeback(
     struct hvm_emulate_ctxt *hvmemul_ctxt);
+int hvmemul_cpuid(
+    unsigned int *eax,
+    unsigned int *ebx,
+    unsigned int *ecx,
+    unsigned int *edx,
+    struct x86_emulate_ctxt *ctxt);
 struct segment_register *hvmemul_get_seg_reg(
     enum x86_segment seg,
     struct hvm_emulate_ctxt *hvmemul_ctxt);
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -504,6 +504,8 @@ extern int mmcfg_intercept_write(enum x8
                                  void *p_data,
                                  unsigned int bytes,
                                  struct x86_emulate_ctxt *ctxt);
+int pv_emul_cpuid(unsigned int *eax, unsigned int *ebx, unsigned int *ecx,
+                  unsigned int *edx, struct x86_emulate_ctxt *ctxt);
 
 int  ptwr_do_page_fault(struct vcpu *, unsigned long,
                         struct cpu_user_regs *);