[Xen-devel] [PATCH v2 3/3] x86/emul: Support CPUID faulting via a speculative MSR read

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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>

v2:
 * Substantial rebase
 * Reimplement speculative reading by squashing the exception rather than
   trying to prevent it by using an extra function parameter.
---
 xen/arch/x86/hvm/emulate.c             |  9 ---------
 xen/arch/x86/traps.c                   | 18 +-----------------
 xen/arch/x86/x86_emulate/x86_emulate.c | 19 +++++++++++++++++--
 xen/arch/x86/x86_emulate/x86_emulate.h |  7 +------
 4 files changed, 19 insertions(+), 34 deletions(-)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index edcae5e..f24d289 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1575,15 +1575,6 @@ static int hvmemul_wbinvd(
 int hvmemul_cpuid(uint32_t leaf, uint32_t subleaf,
                   struct cpuid_leaf *res, 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;
-
     guest_cpuid(current, leaf, subleaf, res);
     return X86EMUL_OKAY;
 }
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index ec8b002..75c89eb 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2884,23 +2884,7 @@ static int priv_op_wbinvd(struct x86_emulate_ctxt *ctxt)
 int pv_emul_cpuid(uint32_t leaf, uint32_t subleaf,
                   struct cpuid_leaf *res, struct x86_emulate_ctxt *ctxt)
 {
-    const struct vcpu *curr = current;
-
-    /*
-     * x86_emulate uses this function to query CPU features for its own
-     * internal use. Make sure we're actually emulating CPUID before checking
-     * for emulated CPUID faulting.
-     */
-    if ( ctxt->opcode == X86EMUL_OPC(0x0f, 0xa2) )
-    {
-
-        /* If cpuid faulting is enabled and CPL>0 leave the #GP untouched. */
-        if ( curr->arch.cpuid_faulting &&
-             !guest_kernel_mode(curr, ctxt->regs) )
-            return X86EMUL_EXCEPTION;
-    }
-
-    guest_cpuid(curr, leaf, subleaf, res);
+    guest_cpuid(current, leaf, subleaf, res);
 
     return X86EMUL_OKAY;
 }
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index f339d36..c3fc26a 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -5424,10 +5424,25 @@ x86_emulate(
         break;
 
     case X86EMUL_OPC(0x0f, 0xa2): /* cpuid */
+        msr_val = 0;
         fail_if(ops->cpuid == NULL);
+
+        /* Speculatively read MSR_INTEL_MISC_FEATURES_ENABLES. */
+        if ( ops->read_msr &&
+             (rc = ops->read_msr(MSR_INTEL_MISC_FEATURES_ENABLES,
+                                 &msr_val, ctxt)) == X86EMUL_EXCEPTION )
+        {
+            /* Not implemented.  Squash the exception and proceed normally. */
+            x86_emul_reset_event(ctxt);
+            rc = X86EMUL_OKAY;
+        }
+        if ( rc != X86EMUL_OKAY )
+            goto done;
+
+        generate_exception_if((msr_val & MSR_MISC_FEATURES_CPUID_FAULTING) &&
+                              !mode_ring0(), EXC_GP, 0); /* Faulting active? */
+
         rc = ops->cpuid(_regs._eax, _regs._ecx, &cpuid_leaf, ctxt);
-        generate_exception_if(rc == X86EMUL_EXCEPTION,
-                              EXC_GP, 0); /* CPUID Faulting? */
         if ( rc != X86EMUL_OKAY )
             goto done;
         _regs.r(ax) = cpuid_leaf.a;
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.h 
b/xen/arch/x86/x86_emulate/x86_emulate.h
index 071668d..c35873e 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -413,12 +413,7 @@ 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)(
         uint32_t leaf,
         uint32_t subleaf,
-- 
2.1.4


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