[Xen-devel] [PATCH 08/10] x86/cpuid: Improvements to guest policies for speculative sidechannel features

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

If Xen isn't virtualising MSR_SPEC_CTRL for guests, IBRSB shouldn't be
advertised.  It is not currently possible to express this via the existing
command line options, but such an ability will be introduced.

Another useful option in some usecases is to offer IBPB without IBRS.  When a
guest kernel is known to be compatible (uses retpoline and knows about the AMD
IBPB feature bit), an administrator with pre-Skylake hardware may wish to hide
IBRS.  This allows the VM to have full protection, without Xen or the VM
needing to touch MSR_SPEC_CTRL, which can reduce the overhead of Spectre
mitigations.

Break the logic common to both PV and HVM CPUID calculations into a common
helper, to avoid duplication.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
---
 xen/arch/x86/cpuid.c | 60 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 37 insertions(+), 23 deletions(-)

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index cff1a26..827b6c5 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -368,6 +368,28 @@ static void __init calculate_host_policy(void)
     }
 }
 
+static void __init guest_common_feature_adjustments(uint32_t *fs)
+{
+    /* Unconditionally claim to be able to set the hypervisor bit. */
+    __set_bit(X86_FEATURE_HYPERVISOR, fs);
+
+    /*
+     * If IBRS is offered to the guest, unconditionally offer STIBP.  It is a
+     * nop on non-HT hardware, and has this behaviour to make heterogeneous
+     * setups easier to manage.
+     */
+    if ( test_bit(X86_FEATURE_IBRSB, fs) )
+        __set_bit(X86_FEATURE_STIBP, fs);
+
+    /*
+     * On hardware which supports IBRS/IBPB, we can offer IBPB independently
+     * of IBRS by using the AMD feature bit.  An administrator may wish for
+     * performance reasons to offer IBPB without IBRS.
+     */
+    if ( host_cpuid_policy.feat.ibrsb )
+        __set_bit(X86_FEATURE_IBPB, fs);
+}
+
 static void __init calculate_pv_max_policy(void)
 {
     struct cpuid_policy *p = &pv_max_cpuid_policy;
@@ -380,18 +402,14 @@ static void __init calculate_pv_max_policy(void)
     for ( i = 0; i < ARRAY_SIZE(pv_featureset); ++i )
         pv_featureset[i] &= pv_featuremask[i];
 
-    /* Unconditionally claim to be able to set the hypervisor bit. */
-    __set_bit(X86_FEATURE_HYPERVISOR, pv_featureset);
-
-    /* On hardware with IBRS/IBPB support, there are further adjustments. */
-    if ( test_bit(X86_FEATURE_IBRSB, pv_featureset) )
-    {
-        /* Offer STIBP unconditionally.  It is a nop on non-HT hardware. */
-        __set_bit(X86_FEATURE_STIBP, pv_featureset);
+    /*
+     * If Xen isn't virtualising MSR_SPEC_CTRL for PV guests because of
+     * administrator choice, hide the feature.
+     */
+    if ( !boot_cpu_has(X86_FEATURE_SC_MSR_PV) )
+        __clear_bit(X86_FEATURE_IBRSB, pv_featureset);
 
-        /* AMD's IBPB is a subset of IBRS/IBPB. */
-        __set_bit(X86_FEATURE_IBPB, pv_featureset);
-    }
+    guest_common_feature_adjustments(pv_featureset);
 
     sanitise_featureset(pv_featureset);
     cpuid_featureset_to_policy(pv_featureset, p);
@@ -419,9 +437,6 @@ static void __init calculate_hvm_max_policy(void)
     for ( i = 0; i < ARRAY_SIZE(hvm_featureset); ++i )
         hvm_featureset[i] &= hvm_featuremask[i];
 
-    /* Unconditionally claim to be able to set the hypervisor bit. */
-    __set_bit(X86_FEATURE_HYPERVISOR, hvm_featureset);
-
     /*
      * Xen can provide an APIC emulation to HVM guests even if the host's APIC
      * isn't enabled.
@@ -438,6 +453,13 @@ static void __init calculate_hvm_max_policy(void)
         __set_bit(X86_FEATURE_SEP, hvm_featureset);
 
     /*
+     * If Xen isn't virtualising MSR_SPEC_CTRL for HVM guests because of
+     * administrator choice, hide the feature.
+     */
+    if ( !boot_cpu_has(X86_FEATURE_SC_MSR_HVM) )
+        __clear_bit(X86_FEATURE_IBRSB, hvm_featureset);
+
+    /*
      * With VT-x, some features are only supported by Xen if dedicated
      * hardware support is also available.
      */
@@ -450,15 +472,7 @@ static void __init calculate_hvm_max_policy(void)
             __clear_bit(X86_FEATURE_XSAVES, hvm_featureset);
     }
 
-    /* On hardware with IBRS/IBPB support, there are further adjustments. */
-    if ( test_bit(X86_FEATURE_IBRSB, hvm_featureset) )
-    {
-        /* Offer STIBP unconditionally.  It is a nop on non-HT hardware. */
-        __set_bit(X86_FEATURE_STIBP, hvm_featureset);
-
-        /* AMD's IBPB is a subset of IBRS/IBPB. */
-        __set_bit(X86_FEATURE_IBPB, hvm_featureset);
-    }
+    guest_common_feature_adjustments(hvm_featureset);
 
     sanitise_featureset(hvm_featureset);
     cpuid_featureset_to_policy(hvm_featureset, p);
-- 
2.1.4


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