[PATCH v4 2/4] KVM: TDX: Report CORE_CAPABILITIES as configurable

[Binbin Wu]

Add CORE_CAPABILITIES (CPUID.0x7.0.EDX[30]) to KVM's allowlist of TDX
directly configurable CPUID feature bits, even though KVM doesn't support
MSR_IA32_CORE_CAPS for TDX guests, to accommodate the legacy TDX module
definition and userspace's stale knowledge of it.

Older TDX specifications define the CORE_CAPABILITIES CPUID bit as
fixed-1, so userspace may expect the bit to be enabled for TDs.  #VE
reduction turns it into a directly configurable bit, so leaving it out of
the allowlist would make the bit impossible to enable once KVM starts
validating userspace's CPUID input, i.e. would be a surprising behavior
change for such userspace.

Reporting CORE_CAPABILITIES as directly configurable also lets userspace
detect that the bit is no longer fixed-1, and thus correct its stale
knowledge.

Keep MSR_IA32_CORE_CAPS unsupported for TDs, as no existing TDX user needs
guest access to the MSR.

Note, CORE_CAPABILITIES is the only bit that is unsupported by KVM *and*
changed from fixed-1 to directly configurable by #VE reduction, and no
further #VE reductions are expected.

Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
Reviewed-by: Tony Lindgren <tony.lindgren@xxxxxxxxxxxxxxx>
---
v4:
- Add #VE reduction related background to the changelog. (Kishen)
- Add RB from Tony.

v3:
- Drop the code for MSR_IA32_CORE_CAPS access.
---
 arch/x86/kvm/vmx/tdx.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 2b51a85c998e8..b34afc52b714e 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -165,6 +165,13 @@ static void __init tdx_initialize_cpu_cfg_caps(void)
 		TDX_CFG_F(AVX512_VP2INTERSECT),
 		TDX_CFG_F(SERIALIZE),
 		TDX_CFG_F(TSXLDTRK),
+		/*
+		 * KVM doesn't support MSR_IA32_CORE_CAPS, but older TDX specs
+		 * define this bit as fixed-1.  Report it as configurable to
+		 * accommodate the legacy TDX module definition, and to let
+		 * userspace detect that the bit is no longer fixed-1.
+		 */
+		TDX_CFG_EXTRA_F(CORE_CAPABILITIES),
 	);
 
 	tdx_cpu_cfg_cap_init(CPUID_7_1_EAX,
-- 
2.46.0