From: Babu Moger <babu.moger@amd.com>

Transient Scheduler Attacks (TSA) are new speculative side channel attacks
related to the execution timing of instructions under specific
microarchitectural conditions. In some cases, an attacker may be able to
use this timing information to infer data from other contexts, resulting in
information leakage

CPUID Fn8000_0021 EAX[5] (VERW_CLEAR). If this bit is 1, the memory form of
the VERW instruction may be used to help mitigate TSA.

Link: https://www.amd.com/content/dam/amd/en/documents/resources/bulletin/technical-guidance-for-mitigating-transient-scheduler-attacks.pdf
Co-developed-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Babu Moger <babu.moger@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/e6362672e3a67a9df661a8f46598335a1a2d2754.1752176771.git.babu.moger@amd.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/cpu.h | 2 ++
 target/i386/cpu.c | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index f74f13534e7..ce948861a76 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1104,6 +1104,8 @@ uint64_t x86_cpu_get_supported_feature_word(X86CPU *cpu, FeatureWord w);
 #define CPUID_8000_0021_EAX_FS_GS_BASE_NS                (1U << 1)
 /* LFENCE is always serializing */
 #define CPUID_8000_0021_EAX_LFENCE_ALWAYS_SERIALIZING    (1U << 2)
+/* Memory form of VERW mitigates TSA */
+#define CPUID_8000_0021_EAX_VERW_CLEAR                   (1U << 5)
 /* Null Selector Clears Base */
 #define CPUID_8000_0021_EAX_NULL_SEL_CLR_BASE            (1U << 6)
 /* Automatic IBRS */
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 339881b40f8..4f99cbc5c0b 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -1397,7 +1397,7 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
         .type = CPUID_FEATURE_WORD,
         .feat_names = {
             "no-nested-data-bp", "fs-gs-base-ns", "lfence-always-serializing", NULL,
-            NULL, NULL, "null-sel-clr-base", NULL,
+            NULL, "verw-clear", "null-sel-clr-base", NULL,
             "auto-ibrs", NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
-- 
2.51.0