[PATCH 1/2] x86/hvm: Don't emulate all instructions hitting the #UD intercept

[Andrew Cooper <andrew.cooper3@citrix.com>]

Having the instruction emulator fill in all #UDs when using FEP is unhelpful
when trying to test emulation behaviour against hardware.

Restrict emulation from the #UD intercept to the cross-vendor case, and when a
postive Forced Emulation Prefix has been identified.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
---
 xen/arch/x86/hvm/hvm.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 73d24df..12a6f46 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4002,13 +4002,15 @@ int hvm_msr_write_intercept(unsigned int msr, uint64_t msr_content,
 
 void hvm_ud_intercept(struct cpu_user_regs *regs)
 {
+    struct vcpu *cur = current;
+    bool should_emulate =
+        cur->domain->arch.x86_vendor != boot_cpu_data.x86_vendor;
     struct hvm_emulate_ctxt ctxt;
 
     hvm_emulate_init_once(&ctxt, regs);
 
     if ( opt_hvm_fep )
     {
-        struct vcpu *cur = current;
         const struct segment_register *cs = &ctxt.seg_reg[x86_seg_cs];
         uint32_t walk = (ctxt.seg_reg[x86_seg_ss].attr.fields.dpl == 3)
             ? PFEC_user_mode : 0;
@@ -4032,9 +4034,17 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
                 regs->eip = regs->_eip;
 
             add_taint(TAINT_HVM_FEP);
+
+            should_emulate = true;
         }
     }
 
+    if ( !should_emulate )
+    {
+        hvm_inject_hw_exception(TRAP_invalid_op, X86_EVENT_NO_EC);
+        return;
+    }
+
     switch ( hvm_emulate_one(&ctxt) )
     {
     case X86EMUL_UNHANDLEABLE:
-- 
2.1.4


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