As for Spectre variant-2, we rely on SMCCC 1.1 to provide the discovery
mechanism for detecting the SSBD mitigation.

A new capability is also allocated for that purpose, and a config
option.

This is part of XSA-263.

Signed-off-by: Julien Grall <julien.gr...@arm.com>

---
    Changes in v3:
        - required should be false when then mitigation is not required
        on a given CPU

    Changes in v2:
        - Add the switch in this patch rather than the next one.
        - s/supported/required/
---
 xen/arch/arm/Kconfig             | 10 +++++++
 xen/arch/arm/cpuerrata.c         | 58 ++++++++++++++++++++++++++++++++++++++++
 xen/include/asm-arm/cpuerrata.h  | 21 +++++++++++++++
 xen/include/asm-arm/cpufeature.h |  3 ++-
 xen/include/asm-arm/smccc.h      |  7 +++++
 5 files changed, 98 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
index 8174c0c635..0e2d027060 100644
--- a/xen/arch/arm/Kconfig
+++ b/xen/arch/arm/Kconfig
@@ -73,6 +73,16 @@ config SBSA_VUART_CONSOLE
          Allows a guest to use SBSA Generic UART as a console. The
          SBSA Generic UART implements a subset of ARM PL011 UART.
 
+config ARM_SSBD
+       bool "Speculative Store Bypass Disable" if EXPERT = "y"
+       depends on HAS_ALTERNATIVE
+       default y
+       help
+         This enables mitigation of bypassing of previous stores by speculative
+         loads.
+
+         If unsure, say Y.
+
 endmenu
 
 menu "ARM errata workaround via the alternative framework"
diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
index 1baa20654b..1a6130406c 100644
--- a/xen/arch/arm/cpuerrata.c
+++ b/xen/arch/arm/cpuerrata.c
@@ -235,6 +235,58 @@ static int enable_ic_inv_hardening(void *data)
 
 #endif
 
+#ifdef CONFIG_ARM_SSBD
+
+/*
+ * Assembly code may use the variable directly, so we need to make sure
+ * it fits in a register.
+ */
+DEFINE_PER_CPU_READ_MOSTLY(register_t, ssbd_callback_required);
+
+static bool has_ssbd_mitigation(const struct arm_cpu_capabilities *entry)
+{
+    struct arm_smccc_res res;
+    bool required;
+
+    if ( smccc_ver < SMCCC_VERSION(1, 1) )
+        return false;
+
+    /*
+     * The probe function return value is either negative (unsupported
+     * or mitigated), positive (unaffected), or zero (requires
+     * mitigation). We only need to do anything in the last case.
+     */
+    arm_smccc_1_1_smc(ARM_SMCCC_ARCH_FEATURES_FID,
+                      ARM_SMCCC_ARCH_WORKAROUND_2_FID, &res);
+
+    switch ( (int)res.a0 )
+    {
+    case ARM_SMCCC_NOT_SUPPORTED:
+        return false;
+
+    case ARM_SMCCC_NOT_REQUIRED:
+        return false;
+
+    case ARM_SMCCC_SUCCESS:
+        required = true;
+        break;
+
+    case 1: /* Mitigation not required on this CPU. */
+        required = false;
+        break;
+
+    default:
+        ASSERT_UNREACHABLE();
+        return false;
+    }
+
+    if ( required )
+        this_cpu(ssbd_callback_required) = 1;
+
+    return required;
+}
+#endif
+
 #define MIDR_RANGE(model, min, max)     \
     .matches = is_affected_midr_range,  \
     .midr_model = model,                \
@@ -336,6 +388,12 @@ static const struct arm_cpu_capabilities arm_errata[] = {
         .enable = enable_ic_inv_hardening,
     },
 #endif
+#ifdef CONFIG_ARM_SSBD
+    {
+        .capability = ARM_SSBD,
+        .matches = has_ssbd_mitigation,
+    },
+#endif
     {},
 };
 
diff --git a/xen/include/asm-arm/cpuerrata.h b/xen/include/asm-arm/cpuerrata.h
index 4e45b237c8..e628d3ff56 100644
--- a/xen/include/asm-arm/cpuerrata.h
+++ b/xen/include/asm-arm/cpuerrata.h
@@ -27,9 +27,30 @@ static inline bool check_workaround_##erratum(void)          
   \
 
 CHECK_WORKAROUND_HELPER(766422, ARM32_WORKAROUND_766422, CONFIG_ARM_32)
 CHECK_WORKAROUND_HELPER(834220, ARM64_WORKAROUND_834220, CONFIG_ARM_64)
+CHECK_WORKAROUND_HELPER(ssbd, ARM_SSBD, CONFIG_ARM_SSBD)
 
 #undef CHECK_WORKAROUND_HELPER
 
+#ifdef CONFIG_ARM_SSBD
+
+#include <asm/current.h>
+
+DECLARE_PER_CPU(register_t, ssbd_callback_required);
+
+static inline bool cpu_require_ssbd_mitigation(void)
+{
+    return this_cpu(ssbd_callback_required);
+}
+
+#else
+
+static inline bool cpu_require_ssbd_mitigation(void)
+{
+    return false;
+}
+
+#endif
+
 #endif /* __ARM_CPUERRATA_H__ */
 /*
  * Local variables:
diff --git a/xen/include/asm-arm/cpufeature.h b/xen/include/asm-arm/cpufeature.h
index e557a095af..2a5c075d3b 100644
--- a/xen/include/asm-arm/cpufeature.h
+++ b/xen/include/asm-arm/cpufeature.h
@@ -43,8 +43,9 @@
 #define SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT 5
 #define SKIP_CTXT_SWITCH_SERROR_SYNC 6
 #define ARM_HARDEN_BRANCH_PREDICTOR 7
+#define ARM_SSBD 8
 
-#define ARM_NCAPS           8
+#define ARM_NCAPS           9
 
 #ifndef __ASSEMBLY__
 
diff --git a/xen/include/asm-arm/smccc.h b/xen/include/asm-arm/smccc.h
index 8342cc33fe..a6804cec99 100644
--- a/xen/include/asm-arm/smccc.h
+++ b/xen/include/asm-arm/smccc.h
@@ -258,7 +258,14 @@ struct arm_smccc_res {
                       ARM_SMCCC_OWNER_ARCH,         \
                       0x8000)
 
+#define ARM_SMCCC_ARCH_WORKAROUND_2_FID             \
+    ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,         \
+                       ARM_SMCCC_CONV_32,           \
+                       ARM_SMCCC_OWNER_ARCH,        \
+                       0x7FFF)
+
 /* SMCCC error codes */
+#define ARM_SMCCC_NOT_REQUIRED          (-2)
 #define ARM_SMCCC_ERR_UNKNOWN_FUNCTION  (-1)
 #define ARM_SMCCC_NOT_SUPPORTED         (-1)
 #define ARM_SMCCC_SUCCESS               (0)
-- 
2.11.0


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