[Qemu-devel] [PATCH 2/7] target-i386: Don't try to enable PT State xsave component

[Eduardo Habkost <ehabkost@redhat.com>]

The code that calculates the set of supported XSAVE components on
CPUID looks at ext_save_areas to find out which components should
be enabled. However, if there are zeroed entries in the
ext_save_areas array, the
  ((env->features[esa->feature] & esa->bits) == esa->bits)
check will always succeed and QEMU will unconditionally try to
enable the component.

Luckily this never caused any problems because the only missing
entry in ext_save_areas is the PT State component (bit 8), and
KVM currently doesn't support it (so it was cleared on ena_mask).
But the code was still incorrect and would break if KVM starts
returning CPUID[EAX=0xD,ECX=0].EAX[bit 8] as supported on
GET_SUPPORTED_CPUID.

Fix the problem by changing the code to not enable a XSAVE
component if ExtSaveArea::bits is zero.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target-i386/cpu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index cc07fdb..25ab4f8 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -2514,7 +2514,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
             *ecx = 0x240;
             for (i = 2; i < ARRAY_SIZE(x86_ext_save_areas); i++) {
                 const ExtSaveArea *esa = &x86_ext_save_areas[i];
-                if ((env->features[esa->feature] & esa->bits) == esa->bits
+                if ((env->features[esa->feature] & esa->bits)
                     && ((ena_mask >> i) & 1) != 0) {
                     if (i < 32) {
                         *eax |= 1u << i;
@@ -2530,7 +2530,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
             *eax = env->features[FEAT_XSAVE];
         } else if (count < ARRAY_SIZE(x86_ext_save_areas)) {
             const ExtSaveArea *esa = &x86_ext_save_areas[count];
-            if ((env->features[esa->feature] & esa->bits) == esa->bits
+            if ((env->features[esa->feature] & esa->bits)
                 && ((ena_mask >> count) & 1) != 0) {
                 *eax = esa->size;
                 *ebx = esa->offset;
@@ -2766,7 +2766,7 @@ static void x86_cpu_reset(CPUState *s)
     }
     for (i = 2; i < ARRAY_SIZE(x86_ext_save_areas); i++) {
         const ExtSaveArea *esa = &x86_ext_save_areas[i];
-        if ((env->features[esa->feature] & esa->bits) == esa->bits) {
+        if (env->features[esa->feature] & esa->bits) {
             xcr0 |= 1ull << i;
         }
     }
-- 
2.7.4