x86/cpuid: Handling of IBRS/IBPB, STIBP and IBRS for guests

Intel specifies IBRS/IBPB (combined, in a single bit) and STIBP as a separate
bit.  AMD specifies IBPB alone in a 3rd bit.

AMD's IBPB is a subset of Intel's combined IBRS/IBPB.  For performance
reasons, administrators might wish to express "IBPB only" even on Intel
hardware, so we allow the AMD bit to be used for this purpose.

The behaviour of STIBP is more complicated.

It is our current understanding that STIBP will be advertised on HT-capable
hardware irrespective of whether HT is enabled, but not advertised on
HT-incapable hardware.  However, for ease of virtualisation, STIBP's
functionality is ignored rather than reserved by microcode/hardware on
HT-incapable hardware.

For guest safety, we treat STIBP as special, always override the toolstack
choice, and always advertise STIBP if IBRS is available.  This removes the
corner case where STIBP is not advertised, but the guest is running on
HT-capable hardware where it does matter.

Finally as a bugfix, update the libxc CPUID logic to understand the e8b
feature leaf, which has the side effect of also offering CLZERO to guests on
applicable hardware.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: d297b56682
master date: 2018-01-26 14:10:21 +0000

xen/arch/x86/hvm/hvm.c

@@ -4596,6 +4596,20 @@ void hvm_cpuid(unsigned int input, unsigned int *eax, unsigned int *ebx,
         /* Don't expose INVPCID to non-hap hvm. */
         if ( (count == 0) && !hap_enabled(d) )
             *ebx &= ~cpufeat_mask(X86_FEATURE_INVPCID);
+
+        if ( count == 0 )
+        {
+            /*
+             * Override STIBP to match IBRS.  Guests can safely use STIBP
+             * functionality on non-HT hardware, but can't necesserily protect
+             * themselves from SP2/Spectre/Branch Target Injection if STIBP is
+             * hidden on HT-capable hardware.
+             */
+            if ( *edx & cpufeat_mask(X86_FEATURE_IBRSB) )
+                *edx |= cpufeat_mask(X86_FEATURE_STIBP);
+            else
+                *edx &= ~cpufeat_mask(X86_FEATURE_STIBP);
+        }
         break;
     case 0xb:
         /* Fix the x2APIC identifier. */
@@ -4650,6 +4664,11 @@ void hvm_cpuid(unsigned int input, unsigned int *eax, unsigned int *ebx,
         hvm_cpuid(0x80000001, NULL, NULL, NULL, &_edx);
         *eax = (*eax & ~0xffff00) | (_edx & cpufeat_mask(X86_FEATURE_LM)
                                      ? 0x3000 : 0x2000);
+
+        /* AMD's IBPB is a subset of IBRS/IBPB. */
+        hvm_cpuid(7, NULL, &_ebx, NULL, NULL);
+        if ( _ebx & cpufeat_mask(X86_FEATURE_IBRSB) )
+            *ebx |= cpufeat_mask(X86_FEATURE_IBPB);
         break;
     }
 }

xen/arch/x86/traps.c

@@ -843,9 +843,10 @@ void pv_cpuid(struct cpu_user_regs *regs)
 
         switch ( cpuid_leaf )
         {
+            unsigned int _eax, _ebx, _ecx, _edx;
+
         case XSTATE_CPUID:
         {
-            unsigned int _eax, _ebx, _ecx, _edx;
             /* EBX value of main leaf 0 depends on enabled xsave features */
             if ( sub_leaf == 0 && curr->arch.xcr0 )
             {
@@ -863,6 +864,29 @@ void pv_cpuid(struct cpu_user_regs *regs)
             }
             goto xstate;
         }
+
+        case 0x00000007:
+            if ( regs->_ecx == 0 )
+            {
+                /*
+                 * Override STIBP to match IBRS.  Guests can safely use STIBP
+                 * functionality on non-HT hardware, but can't necesserily protect
+                 * themselves from SP2/Spectre/Branch Target Injection if STIBP is
+                 * hidden on HT-capable hardware.
+                 */
+                if ( d & cpufeat_mask(X86_FEATURE_IBRSB) )
+                    d |= cpufeat_mask(X86_FEATURE_STIBP);
+                else
+                    d &= ~cpufeat_mask(X86_FEATURE_STIBP);
+            }
+            break;
+
+        case 0x80000008:
+            /* AMD's IBPB is a subset of IBRS/IBPB. */
+            domain_cpuid(currd, 7, 0, &_eax, &_ebx, &_ecx, &_edx);
+            if ( _edx & cpufeat_mask(X86_FEATURE_IBRSB) )
+                b |= cpufeat_mask(X86_FEATURE_IBPB);
+            break;
         }
         goto out;
     }
@@ -920,6 +944,7 @@ void pv_cpuid(struct cpu_user_regs *regs)
 
     case 0x00000007:
         if ( regs->_ecx == 0 )
+        {
             b &= (cpufeat_mask(X86_FEATURE_BMI1) |
                   cpufeat_mask(X86_FEATURE_HLE)  |
                   cpufeat_mask(X86_FEATURE_AVX2) |
@@ -929,9 +954,18 @@ void pv_cpuid(struct cpu_user_regs *regs)
                   cpufeat_mask(X86_FEATURE_RDSEED)  |
                   cpufeat_mask(X86_FEATURE_ADX)  |
                   cpufeat_mask(X86_FEATURE_FSGSBASE));
+
+            d &= cpufeat_mask(X86_FEATURE_IBRSB);
+
+            /* Override STIBP to match IBRS (see above). */
+            if ( d & cpufeat_mask(X86_FEATURE_IBRSB) )
+                d |= cpufeat_mask(X86_FEATURE_STIBP);
+            else
+                d &= ~cpufeat_mask(X86_FEATURE_STIBP);
+        }
         else
-            b = 0;
-        a = c = d = 0;
+            b = d = 0;
+        a = c = 0;
         break;
 
     case XSTATE_CPUID:
@@ -975,6 +1009,12 @@ void pv_cpuid(struct cpu_user_regs *regs)
         __clear_bit(X86_FEATURE_MWAITX % 32, &c);
         break;
 
+    case 0x80000008:
+        /* AMD's IBPB is a subset of IBRS/IBPB. */
+        if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+            b |= cpufeat_mask(X86_FEATURE_IBPB);
+        break;
+
     case 0x0000000a: /* Architectural Performance Monitor Features (Intel) */
         break;