Re: [PATCH v5 4/5] KVM: TDX: Report only KVM-allowed CPUIDs in KVM_TDX_CAPABILITIES

[Chao Gao]

On Fri, Oct 16, 2026 at 05:14:21PM +0800, Binbin Wu wrote:
>Filter the directly configurable CPUID bits reported through
>KVM_TDX_CAPABILITIES against KVM's TDX allowlist, and drop the hardcoded
>denylist-based filtering.
>
>The TDX module reports the CPUID bits that the VMM can directly configure
>for a TD, but KVM must not advertise bits that it doesn't support, as
>blindly exposing a feature that clobbers host state can corrupt the host.
>The existing denylist, which clears only TSX and WAITPKG, can't account
>for such features introduced by future TDX modules.
>
>Add tdx_get_cpuid_cfg_mask() to compute the mask of directly configurable
>bits that KVM allows for a given CPUID register, and apply it to every
>CPUID entry reported through KVM_TDX_CAPABILITIES.  The mask combines
>feature bits, taken from tdx_cpu_cfg_caps[], with non-feature bits, which
>are identified by function, index, and register.
>
>Treat CPUID.0x1.{EAX,EBX}, CPUID.{0x4,0x18,0x1F}, and CPUID.0x80000008.EAX
>as fully allowed, as these registers enumerate non-feature information,
>e.g. family/model/stepping, brand index, CLFLUSH line size, APIC ID,
>cache/TLB/topology properties, and address sizes.  Allow the reserved bits
>in them as well.  Though there's no architectural guarantee, given what
>these registers enumerate, the reserved bits are expected to be used to
>extend the existing information rather than to enumerate new features.
>
>Also allow CPUID.0x24.0.EBX[7:0] (AVX10 version), which the TDX module
>reports as directly configurable when AVX10 is supported, even though the
>CPUID virtualization table in the "Intel TDX ABI Definitions" currently
>lists it as not configurable.  The table is expected to be corrected.
>
>As a result, directly configurable bits that aren't in the allowlist,
>including bits introduced by future TDX modules, aren't advertised to
>userspace until KVM explicitly opts in.
>
>Note, the change is user-visible, as some bits that were previously
>reported as configurable no longer are.  The ABI itself is unchanged: KVM
>already filtered unsupported CPUID bits out of KVM_TDX_CAPABILITIES, albeit
>via a denylist, and the set of configurable bits already varies with the
>TDX module and KVM versions.
>
>The change isn't expected to break userspace or guests:
> - Userspace that honors KVM_TDX_CAPABILITIES simply stops setting the
>   dropped bits, and KVM_TDX_INIT_VM still succeeds.
> - CORE_CAPABILITIES, which older TDX module specs defined as fixed-1, is
>   still allowed.
> - The dropped features are either obsolete or unsupported by KVM, and in
>   the latter case, a guest relying on them would already have run into
>   problems.
>
>Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>