[Xen-devel] [PATCH 7/8] x86/emul: Support CPUID fauilting via a speculative MSR read

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Use the new speculative read support to query MSR_INTEL_MISC_FEATURES for
active CPUID faulting, without raising #GP as a side effect.

This removes the need for every cpuid() emulation hook to individually support
CPUID faulting.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Paul Durrant <paul.durrant@xxxxxxxxxx>

Jan: Probably worth waiting to rebase over your changes moving the
architectrual defines elsewhere
---
 xen/arch/x86/hvm/emulate.c             |  9 ---------
 xen/arch/x86/x86_emulate/x86_emulate.c | 16 +++++++++++++---
 xen/arch/x86/x86_emulate/x86_emulate.h |  7 +------
 3 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index bce0b00..1a132e7 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1567,15 +1567,6 @@ int hvmemul_cpuid(
     unsigned int *edx,
     struct x86_emulate_ctxt *ctxt)
 {
-    /*
-     * x86_emulate uses this function to query CPU features for its own 
internal
-     * use. Make sure we're actually emulating CPUID before emulating CPUID
-     * faulting.
-     */
-    if ( ctxt->opcode == X86EMUL_OPC(0x0f, 0xa2) &&
-         hvm_check_cpuid_faulting(current) )
-        return X86EMUL_EXCEPTION;
-
     hvm_funcs.cpuid_intercept(eax, ebx, ecx, edx);
     return X86EMUL_OKAY;
 }
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 5cba7ec..67495eb 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -406,6 +406,8 @@ typedef union {
 
 /* MSRs. */
 #define MSR_TSC          0x00000010
+#define MSR_INTEL_MISC_FEATURES 0x00000140
+#define MISC_FEATURES_CPUID_FAULTING (1 << 0)
 #define MSR_SYSENTER_CS  0x00000174
 #define MSR_SYSENTER_ESP 0x00000175
 #define MSR_SYSENTER_EIP 0x00000176
@@ -5044,13 +5046,21 @@ x86_emulate(
         src.val = x86_seg_fs;
         goto pop_seg;
 
-    case X86EMUL_OPC(0x0f, 0xa2): /* cpuid */ {
+    case X86EMUL_OPC(0x0f, 0xa2): /* cpuid */
+    {
         unsigned int eax = _regs.eax, ebx = _regs.ebx;
         unsigned int ecx = _regs.ecx, edx = _regs.edx;
+        uint64_t misc_features;
+
         fail_if(ops->cpuid == NULL);
+        generate_exception_if(
+            ops->read_msr &&
+            ops->read_msr(MSR_INTEL_MISC_FEATURES,
+                          &misc_features, true, ctxt) == X86EMUL_OKAY &&
+            (misc_features & MISC_FEATURES_CPUID_FAULTING) &&
+            !mode_ring0(), EXC_GP, 0); /* CPUID Faulting? */
+
         rc = ops->cpuid(&eax, &ebx, &ecx, &edx, ctxt);
-        generate_exception_if(rc == X86EMUL_EXCEPTION,
-                              EXC_GP, 0); /* CPUID Faulting? */
         if ( rc != X86EMUL_OKAY )
             goto done;
         _regs.eax = eax; _regs.ebx = ebx;
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.h 
b/xen/arch/x86/x86_emulate/x86_emulate.h
index 89cf20d..46c863f 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -395,12 +395,7 @@ struct x86_emulate_ops
     int (*wbinvd)(
         struct x86_emulate_ctxt *ctxt);
 
-    /*
-     * cpuid: Emulate CPUID via given set of EAX-EDX inputs/outputs.
-     *
-     * May return X86EMUL_EXCEPTION, which causes the emulator to inject
-     * #GP[0].  Used to implement CPUID faulting.
-     */
+    /* cpuid: Emulate CPUID via given set of EAX-EDX inputs/outputs. */
     int (*cpuid)(
         unsigned int *eax,
         unsigned int *ebx,
-- 
2.1.4


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