[Qemu-devel] [PATCH 0/2] i386: "unavailable-features" QOM property

[Eduardo Habkost]

Currently, libvirt uses the "filtered-features" QOM property at
runtime to ensure no feature was accidentally disabled on VCPUs
because it's not available on the host.

However, the code for "feature-words" assumes that all missing
features have a corresponding CPUID bit, which is not true for
MSR-based features like the ones at FEAT_ARCH_CAPABILITIES.

We could extend X86CPUFeatureWordInfo to include information
about MSR features, but it's impossible to do that while keeping
compatibility with clients that (reasonably) expect all elements
of "filtered-features" to have the cpuid-* fields.

We have a field in "query-cpu-definitions" that already describes
all features that are missing on a CPU, including MSR features:
CpuDefinitionInfo.unavailable-features.  The existing code for
building the unavailable-features array even uses
X86CPU::filtered_features to build the feature list.

This series adds a "unavailable-features" QOM property to X86CPU
objects that have the same semantics of "unavailable-features" on
query-cpu-definitions.  The new property has the same goal of
"filtered-features", but is generic enough to let any kind of CPU
feature to be listed there without relying on low level details
like CPUID leaves or MSR numbers.

Eduardo Habkost (2):
  i386: x86_cpu_list_feature_names() function
  i386: "unavailable-features" QOM property

 target/i386/cpu.c | 55 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 42 insertions(+), 13 deletions(-)

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2.18.0.rc1.1.g3f1ff2140