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

From: Jiri Denemark
Subject: Re: [Qemu-devel] [PATCH 0/2] i386: "unavailable-features" QOM property
Date: Thu, 9 May 2019 15:35:37 +0200
User-agent: Mutt/1.11.4 (2019-03-13)

On Mon, Apr 22, 2019 at 20:47:40 -0300, Eduardo Habkost wrote:
> 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.


Would this unavailable-features property contain only canonical names of
the features or all possible aliases of all features? For example,
"tsc-adjust" can also be spelled as "tsc_adjust". When calling
query-cpu-model-expansion, we have a way to request all variants by
running full expansion on the result of a previous static expansion. Can
we get something like this for unavailable-features too?

As you mentioned, there are two interesting QOM properties:
filtered-features and feature-words and both are used by libvirt. We use
feature-words to get CPU features which were enabled in the guest CPU on
top of what we expected. This is the case of, e.g., a feature added to a
given CPU model for new machine types. I guess we could switch to
checking QOM properties for individual features as a replacement for
feature-words which covers both CPUID and MSR features.


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