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[Xen-devel] [PATCH 2/5] x86/cpu: Undo BIOS CPUID max_leaf limit before querying for features.

If IA32_MISC_ENABLE[22] is set by the BIOS, CPUID.0.EAX will be limited to 3.
Lift this limit before considering whether to query CPUID.7[ECX=0].EBX for

Without this change, dom0 is able to see this feature leaf (as the limit was
subsequently lifted), and will set features appropriately in HVM domain cpuid

The specific bug XenServer observed was the advertisement of the FSGSBASE
feature, but an inability to set CR4.FSGSBASE as Xen considered the bit to be
reserved as cpu_has_fsgsbase incorrectly evaluated as false.

This is a regression introduced by c/s 44e24f8567 "x86: don't call
generic_identify() redundantly" where the redundant call actually resampled
CPUID.7[ECX=0] properly to obtain the feature flags.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>


This fix should be backported as far as Xen 4.2
 xen/arch/x86/cpu/common.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 56c552c..a3bc5fd 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -250,14 +250,15 @@ static void __cpuinit generic_identify(struct cpuinfo_x86 
                        paddr_bits = cpuid_eax(0x80000008) & 0xff;
+       /* Might lift BIOS max_leaf=3 limit. */
+       early_intel_workaround(c);
        /* Intel-defined flags: level 0x00000007 */
        if ( c->cpuid_level >= 0x00000007 ) {
                u32 dummy;
                cpuid_count(0x00000007, 0, &dummy, &ebx, &dummy, &dummy);
                c->x86_capability[X86_FEATURE_FSGSBASE / 32] = ebx;
-       early_intel_workaround(c);

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