From fb84b9baa665ffa4596fd871537e0544d60e40fc Mon Sep 17 00:00:00 2001
From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Date: Wed, 1 Feb 2023 08:57:59 -0500
Subject: [PATCH] target/i386: add support for FB_CLEAR feature

commit 22e1094ca82d5518c1b69aff3e87c550776ae1eb upstream.

As reported by the Intel's doc:
"FB_CLEAR: The processor will overwrite fill buffer values as part of
MD_CLEAR operations with the VERW instruction.
On these processors, L1D_FLUSH does not overwrite fill buffer values."

If this cpu feature is present in host, allow QEMU to choose whether to
show it to the guest too.
One disadvantage of not exposing it is that the guest will report
a non existing vulnerability in
/sys/devices/system/cpu/vulnerabilities/mmio_stale_data
because the mitigation is present only when the cpu has
        (FLUSH_L1D and MD_CLEAR) or FB_CLEAR
features enabled.

Intel-SIG: commit 22e1094ca82d ("target/i386: add support for FB_CLEAR feature")
Backport support for FB_CLEAR feature

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20230201135759.555607-3-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[ jason: amend commit log ]
Signed-off-by: Jason Zeng <jason.zeng@intel.com>
---
 target/i386/cpu.c | 2 +-
 target/i386/cpu.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 512bec3ca3..8adc84b7f9 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -982,7 +982,7 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
             "ssb-no", "mds-no", "pschange-mc-no", "tsx-ctrl",
             "taa-no", NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
-            NULL, NULL, NULL, NULL,
+            NULL, "fb-clear", NULL, NULL,
             NULL, NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 9e094ef934..d9aac5acd2 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -962,6 +962,7 @@ uint64_t x86_cpu_get_supported_feature_word(FeatureWord w,
 #define MSR_ARCH_CAP_PSCHANGE_MC_NO     (1U << 6)
 #define MSR_ARCH_CAP_TSX_CTRL_MSR       (1U << 7)
 #define MSR_ARCH_CAP_TAA_NO             (1U << 8)
+#define MSR_ARCH_CAP_FB_CLEAR           (1U << 17)
 
 #define MSR_CORE_CAP_SPLIT_LOCK_DETECT  (1U << 5)
 
-- 
2.41.0.windows.1