[PATCH v3 4/4] x86/cpu-policy: Derive RSBA/RRSBA for guest policies

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The RSBA bit, "RSB Alternative", means that the RSB may use alternative
predictors when empty.  From a practical point of view, this mean "Retpoline
not safe".

Enhanced IBRS (officially IBRS_ALL in Intel's docs, previously IBRS_ATT) is a
statement that IBRS is implemented in hardware (as opposed to the form
retrofitted to existing CPUs in microcode).

The RRSBA bit, "Restricted-RSBA", is a combination of RSBA, and the eIBRS
property that predictions are tagged with the mode in which they were learnt.
Therefore, it means "when eIBRS is active, the RSB may fall back to
alternative predictors but restricted to the current prediction mode".  As
such, it's stronger statement than RSBA, but still means "Retpoline not safe".

CPUs are not expected to enumerate both RSBA and RRSBA.

Add feature dependencies for EIBRS and RRSBA.  While technically they're not
linked, absolutely nothing good can come of letting the guest see RRSBA
without EIBRS.  Nor a guest seeing EIBRS without IBRSB.  Furthermore, we use
this dependency to simplify the max derivation logic.

The max policies gets RSBA and RRSBA unconditionally set (with the EIBRS
dependency maybe hiding RRSBA).  We can run any VM, even if it has been told
"somewhere you might run, Retpoline isn't safe".

The default policies are more complicated.  A guest shouldn't see both bits,
but it needs to see one if the current host suffers from any form of RSBA, and
which bit it needs to see depends on whether eIBRS is visible or not.
Therefore, the calculation must be performed after sanitise_featureset().

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>

v3:
 * Minor commit message adjustment.
 * Drop changes to recalculate_cpuid_policy().  Deferred to a later series.

v2:
 * Expand/adjust the comment for the max features.
 * Rewrite the default feature derivation in light of new information.
 * Fix up in recalculate_cpuid_policy() too.
---
 xen/arch/x86/cpu-policy.c                   | 39 +++++++++++++++++++++
 xen/include/public/arch-x86/cpufeatureset.h |  4 +--
 xen/tools/gen-cpuid.py                      |  5 ++-
 3 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index ee256ff5a137..cde7f7605c28 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -423,8 +423,17 @@ static void __init 
guest_common_max_feature_adjustments(uint32_t *fs)
          * Retpoline not safe)", so these need to be visible to a guest in all
          * cases, even when it's only some other server in the pool which
          * suffers the identified behaviour.
+         *
+         * We can always run any VM which has previously (or will
+         * subsequently) run on hardware where Retpoline is not safe.
+         * Note:
+         *  - The dependency logic may hide RRSBA for other reasons.
+         *  - The max policy does not contitute a sensible configuration to
+         *    run a guest in.
          */
         __set_bit(X86_FEATURE_ARCH_CAPS, fs);
+        __set_bit(X86_FEATURE_RSBA, fs);
+        __set_bit(X86_FEATURE_RRSBA, fs);
     }
 }
 
@@ -532,6 +541,21 @@ static void __init calculate_pv_def_policy(void)
     guest_common_default_feature_adjustments(fs);
 
     sanitise_featureset(fs);
+
+    /*
+     * If the host suffers from RSBA of any form, and the guest can see
+     * MSR_ARCH_CAPS, reflect the appropriate RSBA/RRSBA property to the guest
+     * depending on the visibility of eIBRS.
+     */
+    if ( test_bit(X86_FEATURE_ARCH_CAPS, fs) &&
+         (cpu_has_rsba || cpu_has_rrsba) )
+    {
+        bool eibrs = test_bit(X86_FEATURE_EIBRS, fs);
+
+        __set_bit(eibrs ? X86_FEATURE_RRSBA
+                        : X86_FEATURE_RSBA, fs);
+    }
+
     x86_cpu_featureset_to_policy(fs, p);
     recalculate_xstate(p);
 }
@@ -664,6 +688,21 @@ static void __init calculate_hvm_def_policy(void)
         __set_bit(X86_FEATURE_VIRT_SSBD, fs);
 
     sanitise_featureset(fs);
+
+    /*
+     * If the host suffers from RSBA of any form, and the guest can see
+     * MSR_ARCH_CAPS, reflect the appropriate RSBA/RRSBA property to the guest
+     * depending on the visibility of eIBRS.
+     */
+    if ( test_bit(X86_FEATURE_ARCH_CAPS, fs) &&
+         (cpu_has_rsba || cpu_has_rrsba) )
+    {
+        bool eibrs = test_bit(X86_FEATURE_EIBRS, fs);
+
+        __set_bit(eibrs ? X86_FEATURE_RRSBA
+                        : X86_FEATURE_RSBA, fs);
+    }
+
     x86_cpu_featureset_to_policy(fs, p);
     recalculate_xstate(p);
 }
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index ea779c29879e..ce7407d6a10c 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -311,7 +311,7 @@ XEN_CPUFEATURE(CET_SSS,            15*32+18) /*   CET 
Supervisor Shadow Stacks s
 /* Intel-defined CPU features, MSR_ARCH_CAPS 0x10a.eax, word 16 */
 XEN_CPUFEATURE(RDCL_NO,            16*32+ 0) /*A  No Rogue Data Cache Load 
(Meltdown) */
 XEN_CPUFEATURE(EIBRS,              16*32+ 1) /*A  Enhanced IBRS */
-XEN_CPUFEATURE(RSBA,               16*32+ 2) /*!A RSB Alternative (Retpoline 
not safe) */
+XEN_CPUFEATURE(RSBA,               16*32+ 2) /*!  RSB Alternative (Retpoline 
not safe) */
 XEN_CPUFEATURE(SKIP_L1DFL,         16*32+ 3) /*   Don't need to flush L1D on 
VMEntry */
 XEN_CPUFEATURE(INTEL_SSB_NO,       16*32+ 4) /*A  No Speculative Store Bypass 
*/
 XEN_CPUFEATURE(MDS_NO,             16*32+ 5) /*A  No Microarchitectural Data 
Sampling */
@@ -327,7 +327,7 @@ XEN_CPUFEATURE(FBSDP_NO,           16*32+14) /*A  No Fill 
Buffer Stale Data Prop
 XEN_CPUFEATURE(PSDP_NO,            16*32+15) /*A  No Primary Stale Data 
Propagation */
 XEN_CPUFEATURE(FB_CLEAR,           16*32+17) /*A  Fill Buffers cleared by VERW 
*/
 XEN_CPUFEATURE(FB_CLEAR_CTRL,      16*32+18) /*   
MSR_OPT_CPU_CTRL.FB_CLEAR_DIS */
-XEN_CPUFEATURE(RRSBA,              16*32+19) /*!A Restricted RSB Alternative */
+XEN_CPUFEATURE(RRSBA,              16*32+19) /*!  Restricted RSB Alternative */
 XEN_CPUFEATURE(BHI_NO,             16*32+20) /*A  No Branch History Injection  
*/
 XEN_CPUFEATURE(XAPIC_STATUS,       16*32+21) /*   MSR_XAPIC_DISABLE_STATUS */
 XEN_CPUFEATURE(OVRCLK_STATUS,      16*32+23) /*   MSR_OVERCLOCKING_STATUS */
diff --git a/xen/tools/gen-cpuid.py b/xen/tools/gen-cpuid.py
index 973fcc1c64e8..72cf11654ba9 100755
--- a/xen/tools/gen-cpuid.py
+++ b/xen/tools/gen-cpuid.py
@@ -318,7 +318,7 @@ def crunch_numbers(state):
         # IBRSB/IBRS, and we pass this MSR directly to guests.  Treating them
         # as dependent features simplifies Xen's logic, and prevents the guest
         # from seeing implausible configurations.
-        IBRSB: [STIBP, SSBD, INTEL_PSFD],
+        IBRSB: [STIBP, SSBD, INTEL_PSFD, EIBRS],
         IBRS: [AMD_STIBP, AMD_SSBD, PSFD, AUTO_IBRS,
                IBRS_ALWAYS, IBRS_FAST, IBRS_SAME_MODE],
         AMD_STIBP: [STIBP_ALWAYS],
@@ -328,6 +328,9 @@ def crunch_numbers(state):
 
         # The ARCH_CAPS CPUID bit enumerates the availability of the whole 
register.
         ARCH_CAPS: list(range(RDCL_NO, RDCL_NO + 64)),
+
+        # The behaviour described by RRSBA depend on eIBRS being active.
+        EIBRS: [RRSBA],
     }
 
     deep_features = tuple(sorted(deps.keys()))
-- 
2.30.2