[PATCH v1 1/2] x86/cpuid: Expose NullSelectorClearsBase CPUID bit to guests

[Jane Malalane <jane.malalane@xxxxxxxxxx>]

AMD Zen3 adds the NullSelectorClearsBase bit to indicate that loading
a NULL segment selector zeroes the base and limit fields, as well as
just attributes.

Expose bit to all guests.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jane Malalane <jane.malalane@xxxxxxxxxx>
---
CC: Wei Liu <wl@xxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
CC: Pu Wen <puwen@xxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxx>
---
 tools/libs/light/libxl_cpuid.c              | 1 +
 tools/misc/xen-cpuid.c                      | 1 +
 xen/include/public/arch-x86/cpufeatureset.h | 1 +
 3 files changed, 3 insertions(+)

diff --git a/tools/libs/light/libxl_cpuid.c b/tools/libs/light/libxl_cpuid.c
index b2c673841a..d667c36f31 100644
--- a/tools/libs/light/libxl_cpuid.c
+++ b/tools/libs/light/libxl_cpuid.c
@@ -289,6 +289,7 @@ int libxl_cpuid_parse_config(libxl_cpuid_policy_list 
*cpuid, const char* str)
         {"svm_pausefilt",0x8000000a, NA, CPUID_REG_EDX, 10,  1},
 
         {"lfence+",      0x80000021, NA, CPUID_REG_EAX,  2,  1},
+        {"nscb",         0x80000021, NA, CPUID_REG_EAX,  6,  1},
 
         {"maxhvleaf",    0x40000000, NA, CPUID_REG_EAX,  0,  8},
 
diff --git a/tools/misc/xen-cpuid.c b/tools/misc/xen-cpuid.c
index 735bcf8f0e..d79e67ecfb 100644
--- a/tools/misc/xen-cpuid.c
+++ b/tools/misc/xen-cpuid.c
@@ -185,6 +185,7 @@ static const char *const str_7a1[32] =
 static const char *const str_e21a[32] =
 {
     [ 2] = "lfence+",
+    [ 6] = "nscb",
 };
 
 static const struct {
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index 380b51b1b3..e5a7c94c78 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -285,6 +285,7 @@ XEN_CPUFEATURE(FSRCS,        10*32+12) /*A  Fast Short REP 
CMPSB/SCASB */
 
 /* AMD-defined CPU features, CPUID level 0x80000021.eax, word 11 */
 XEN_CPUFEATURE(LFENCE_DISPATCH,    11*32+ 2) /*A  LFENCE always serializing */
+XEN_CPUFEATURE(NSCB,               11*32+ 6) /*A  Null Selector Clears Base */
 
 #endif /* XEN_CPUFEATURE */
 
-- 
2.11.0