[PATCH v2 2/4] x86/hvm: Disable cross-vendor handling in #UD handler

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

Remove cross-vendor support now that VMs can no longer have a different
vendor than the host.

While at it, refactor the function to exit early and skip initialising
the emulation context when FEP is not enabled.

No functional change intended.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
---
The diff is annoying, but keeping prior logic is silly when it can be
simplified and the emulation context initialisation removed on the
overwhelming majority of cases.

v2:
  * Fix bug introduced in v1: Don't emulate instructions when they
    shouldn't be emulated.
  * Refactor the function so it's simpler.
---
 xen/arch/x86/hvm/hvm.c     | 77 +++++++++++++++-----------------------
 xen/arch/x86/hvm/svm/svm.c |  3 +-
 xen/arch/x86/hvm/vmx/vmx.c |  3 +-
 3 files changed, 32 insertions(+), 51 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 4d37a93c57..8708af9425 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3832,69 +3832,47 @@ int hvm_descriptor_access_intercept(uint64_t exit_info,
     return X86EMUL_OKAY;
 }
 
-static bool cf_check is_cross_vendor(
-    const struct x86_emulate_state *state, const struct x86_emulate_ctxt *ctxt)
-{
-    switch ( ctxt->opcode )
-    {
-    case X86EMUL_OPC(0x0f, 0x05): /* syscall */
-    case X86EMUL_OPC(0x0f, 0x34): /* sysenter */
-    case X86EMUL_OPC(0x0f, 0x35): /* sysexit */
-        return true;
-    }
-
-    return false;
-}
-
 void hvm_ud_intercept(struct cpu_user_regs *regs)
 {
     struct vcpu *cur = current;
-    bool should_emulate =
-        cur->domain->arch.cpuid->x86_vendor != boot_cpu_data.x86_vendor;
     struct hvm_emulate_ctxt ctxt;
+    const struct segment_register *cs = &ctxt.seg_reg[x86_seg_cs];
+    uint32_t walk = PFEC_insn_fetch;
+    unsigned long addr;
+    char sig[5]; /* ud2; .ascii "xen" */
 
-    hvm_emulate_init_once(&ctxt, opt_hvm_fep ? NULL : is_cross_vendor, regs);
+    if ( !opt_hvm_fep )
+        goto reinject;
 
-    if ( opt_hvm_fep )
-    {
-        const struct segment_register *cs = &ctxt.seg_reg[x86_seg_cs];
-        uint32_t walk = ((ctxt.seg_reg[x86_seg_ss].dpl == 3)
-                         ? PFEC_user_mode : 0) | PFEC_insn_fetch;
-        unsigned long addr;
-        char sig[5]; /* ud2; .ascii "xen" */
-
-        if ( hvm_virtual_to_linear_addr(x86_seg_cs, cs, regs->rip,
-                                        sizeof(sig), hvm_access_insn_fetch,
-                                        cs, &addr) &&
-             (hvm_copy_from_guest_linear(sig, addr, sizeof(sig),
-                                         walk, NULL) == HVMTRANS_okay) &&
-             (memcmp(sig, "\xf\xb" "xen", sizeof(sig)) == 0) )
-        {
-            regs->rip += sizeof(sig);
-            regs->eflags &= ~X86_EFLAGS_RF;
+    hvm_emulate_init_once(&ctxt, NULL, regs);
 
-            /* Zero the upper 32 bits of %rip if not in 64bit mode. */
-            if ( !(hvm_long_mode_active(cur) && cs->l) )
-                regs->rip = (uint32_t)regs->rip;
+    if ( ctxt.seg_reg[x86_seg_ss].dpl == 3 )
+        walk |= PFEC_user_mode;
 
-            add_taint(TAINT_HVM_FEP);
+    if ( hvm_virtual_to_linear_addr(x86_seg_cs, cs, regs->rip,
+                                    sizeof(sig), hvm_access_insn_fetch,
+                                    cs, &addr) &&
+         (hvm_copy_from_guest_linear(sig, addr, sizeof(sig),
+                                     walk, NULL) == HVMTRANS_okay) &&
+         (memcmp(sig, "\xf\xb" "xen", sizeof(sig)) == 0) )
+    {
+        regs->rip += sizeof(sig);
+        regs->eflags &= ~X86_EFLAGS_RF;
 
-            should_emulate = true;
-        }
-    }
+        /* Zero the upper 32 bits of %rip if not in 64bit mode. */
+        if ( !(hvm_long_mode_active(cur) && cs->l) )
+            regs->rip = (uint32_t)regs->rip;
 
-    if ( !should_emulate )
-    {
-        hvm_inject_hw_exception(X86_EXC_UD, X86_EVENT_NO_EC);
-        return;
+        add_taint(TAINT_HVM_FEP);
     }
+    else
+        goto reinject;
 
     switch ( hvm_emulate_one(&ctxt, VIO_no_completion) )
     {
     case X86EMUL_UNHANDLEABLE:
     case X86EMUL_UNIMPLEMENTED:
-        hvm_inject_hw_exception(X86_EXC_UD, X86_EVENT_NO_EC);
-        break;
+        goto reinject;
     case X86EMUL_EXCEPTION:
         hvm_inject_event(&ctxt.ctxt.event);
         /* fall through */
@@ -3902,6 +3880,11 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
         hvm_emulate_writeback(&ctxt);
         break;
     }
+
+    return;
+
+ reinject:
+    hvm_inject_hw_exception(X86_EXC_UD, X86_EVENT_NO_EC);
 }
 
 enum hvm_intblk hvm_interrupt_blocked(struct vcpu *v, struct hvm_intack intack)
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 18ba837738..10d1bf350c 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -589,8 +589,7 @@ static void cf_check svm_cpuid_policy_changed(struct vcpu 
*v)
     const struct cpu_policy *cp = v->domain->arch.cpu_policy;
     u32 bitmap = vmcb_get_exception_intercepts(vmcb);
 
-    if ( opt_hvm_fep ||
-         (v->domain->arch.cpuid->x86_vendor != boot_cpu_data.x86_vendor) )
+    if ( opt_hvm_fep )
         bitmap |= (1U << X86_EXC_UD);
     else
         bitmap &= ~(1U << X86_EXC_UD);
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 82c55f49ae..eda99e268d 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -803,8 +803,7 @@ static void cf_check vmx_cpuid_policy_changed(struct vcpu 
*v)
     const struct cpu_policy *cp = v->domain->arch.cpu_policy;
     int rc = 0;
 
-    if ( opt_hvm_fep ||
-         (v->domain->arch.cpuid->x86_vendor != boot_cpu_data.x86_vendor) )
+    if ( opt_hvm_fep )
         v->arch.hvm.vmx.exception_bitmap |= (1U << X86_EXC_UD);
     else
         v->arch.hvm.vmx.exception_bitmap &= ~(1U << X86_EXC_UD);
-- 
2.43.0