According to Chapter "CPUID Virtualization" in TDX module spec, CPUID
bits of TD can be classified into 6 types:
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1 | As configured | configurable by VMM, independent of native value;
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2 | As configured | configurable by VMM if the bit is supported natively
(if native) | Otherwise it equals as native(0).
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3 | Fixed | fixed to 0/1
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4 | Native | reflect the native value
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5 | Calculated | calculated by TDX module.
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6 | Inducing #VE | get #VE exception
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Note:
1. All the configurable XFAM related features and TD attributes related
features fall into type #2. And fixed0/1 bits of XFAM and TD
attributes fall into type #3.
2. For CPUID leaves not listed in "CPUID virtualization Overview" table
in TDX module spec. When they are queried, TDX module injects #VE to
TDs. For this case, TDs can request CPUID emulation from VMM via
TDVMCALL and the values are fully controlled by VMM.
Due to TDX module has its own virtualization policy on CPUID bits, it
leads
to what reported via KVM_GET_SUPPORTED_CPUID diverges from the supported
CPUID bits for TDS. In order to keep a consistent CPUID configuration
between VMM and TDs. Adjust supported CPUID for TDs based on TDX
restrictions.
Currently only focus on the CPUID leaves recognized by QEMU's
feature_word_info[] that are indexed by a FeatureWord.
Introduce a TDX CPUID lookup table, which maintains 1 entry for each
FeatureWord. Each entry has below fields:
- tdx_fixed0/1: The bits that are fixed as 0/1;
- vmm_fixup: The bits that are configurable from the view of TDX
module.
But they requires emulation of VMM when they are
configured
as enabled. For those, they are not supported if VMM doesn't
report them as supported. So they need be fixed up by
checking if VMM supports them.
- inducing_ve: TD gets #VE when querying this CPUID leaf. The result is
totally configurable by VMM.
- supported_on_ve: It's valid only when @inducing_ve is true. It
represents
the maximum feature set supported that be emulated
for TDs.
By applying TDX CPUID lookup table and TDX capabilities reported from
TDX module, the supported CPUID for TDs can be obtained from following
steps:
- get the base of VMM supported feature set;
- if the leaf is not a FeatureWord just return VMM's value without
modification;
- if the leaf is an inducing_ve type, applying supported_on_ve mask and
return;
- include all native bits, it covers type #2, #4, and parts of type #1.
(it also includes some unsupported bits. The following step will
correct it.)
- apply fixed0/1 to it (it covers #3, and rectifies the previous step);
- add configurable bits (it covers the other part of type #1);
- fix the ones in vmm_fixup;
- filter the one has valid .supported field;