Re: [PATCH RFC 1/4] docs: add a table showing x86-64 ABI compatibility levels

[Florian Weimer <fweimer@redhat.com>]

* Peter Maydell:

> Apart from the QEMU/KVM specific CPU models, why is this something
> we should be documenting rather than, say, the people specifying
> what the ABI compatiblity levels are ?

The psABI only cares about userspace, and the specification there
deliberately excludes CPU features used in cryptography blocks, and
features that seem unstable and likely to be disabled in firmware or
future microcode updates.  This seemed a good trade-off for the psABI
because in userspace, run-time detection is still available, to access
additional CPU features not available as part of the target x86-64 level
at build time.

This doesn't really work for virtualization, which also has to decide
what to do with CPU features not relevant to userspace.  And a decision
to exclude features is final in the sense that guests cannot use the
feature at all because run-time detection shows it is as unavailable.

This is why a 1:1 copy of the userspace levels to virtualized machine
models is not possible, I think.

Thanks,
Florian
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