[PATCH 1/7] target/i386: Disable unsupported BTS for guest

[Zide Chen <zide.chen@intel.com>]

BTS (Branch Trace Store), enumerated by IA32_MISC_ENABLE.BTS_UNAVAILABLE
(bit 11), is deprecated and has been superseded by LBR and Intel PT.

KVM yields control of the above mentioned bit to userspace since KVM
commit 9fc222967a39 ("KVM: x86: Give host userspace full control of
MSR_IA32_MISC_ENABLES").

However, QEMU does not set this bit, which allows guests to write the
BTS and BTINT bits in IA32_DEBUGCTL.  Since KVM doesn't support BTS,
this may lead to unexpected MSR access errors.

Setting this bit does not introduce migration compatibility issues, so
the VMState version_id is not bumped.

Signed-off-by: Zide Chen <zide.chen@intel.com>
---
 target/i386/cpu.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 2bbc977d9088..f2b79a8bf1dc 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -474,7 +474,10 @@ typedef enum X86Seg {
 
 #define MSR_IA32_MISC_ENABLE            0x1a0
 /* Indicates good rep/movs microcode on some processors: */
-#define MSR_IA32_MISC_ENABLE_DEFAULT    1
+#define MSR_IA32_MISC_ENABLE_FASTSTRING    1
+#define MSR_IA32_MISC_ENABLE_BTS_UNAVAIL   (1ULL << 11)
+#define MSR_IA32_MISC_ENABLE_DEFAULT       (MSR_IA32_MISC_ENABLE_FASTSTRING     |\
+                                            MSR_IA32_MISC_ENABLE_BTS_UNAVAIL)
 #define MSR_IA32_MISC_ENABLE_MWAIT      (1ULL << 18)
 
 #define MSR_MTRRphysBase(reg)           (0x200 + 2 * (reg))
-- 
2.52.0