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Decimal Floating Point has been available on POWER7 and later (server)
cpus. However, it can be disabled on the hypervisor, meaning that it's
not available to guests.
We currently handle this by conditionally advertising DFP support in the
device tree depending on whether the guest CPU model supports it - which
can also depend on what's allowed in the host for -cpu host. That can lead
to confusion on migration, since host properties are silently affecting
guest visible properties.
This patch handles it by treating it as an optional capability for the
pseries machine type.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
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We currently have some conditionals in the spapr device tree code to decide
whether or not to advertise the availability of the VMX (aka Altivec) and
VSX vector extensions to the guest, based on whether the guest cpu has
those features.
This can lead to confusion and subtle failures on migration, since it makes
a guest visible change based only on host capabilities. We now have a
better mechanism for this, in spapr capabilities flags, which explicitly
depend on user options rather than host capabilities.
Rework the advertisement of VSX and VMX based on a new VSX capability. We
no longer bother with a conditional for VMX support, because every CPU
that's ever been supported by the pseries machine type supports VMX.
NOTE: Some userspace distributions (e.g. RHEL7.4) already rely on
availability of VSX in libc, so using cap-vsx=off may lead to a fatal
SIGILL in init.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
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Now that the "pseries" machine type implements optional capabilities (well,
one so far) there's the possibility of having different capabilities
available at either end of a migration. Although arguably a user error,
it would be nice to catch this situation and fail as gracefully as we can.
This adds code to migrate the capabilities flags. These aren't pulled
directly into the destination's configuration since what the user has
specified on the destination command line should take precedence. However,
they are checked against the destination capabilities.
If the source was using a capability which is absent on the destination,
we fail the migration, since that could easily cause a guest crash or other
bad behaviour. If the source lacked a capability which is present on the
destination we warn, but allow the migration to proceed.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
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This adds an spapr capability bit for Hardware Transactional Memory. It is
enabled by default for pseries-2.11 and earlier machine types. with POWER8
or later CPUs (as it must be, since earlier qemu versions would implicitly
allow it). However it is disabled by default for the latest pseries-2.12
machine type.
This means that with the latest machine type, HTM will not be available,
regardless of CPU, unless it is explicitly enabled on the command line.
That change is made on the basis that:
* This way running with -M pseries,accel=tcg will start with whatever cpu
and will provide the same guest visible model as with accel=kvm.
- More specifically, this means existing make check tests don't have
to be modified to use cap-htm=off in order to run with TCG
* We hope to add a new "HTM without suspend" feature in the not too
distant future which could work on both POWER8 and POWER9 cpus, and
could be enabled by default.
* Best guesses suggest that future POWER cpus may well only support the
HTM-without-suspend model, not the (frankly, horribly overcomplicated)
POWER8 style HTM with suspend.
* Anecdotal evidence suggests problems with HTM being enabled when it
wasn't wanted are more common than being missing when it was.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
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Because PAPR is a paravirtual environment access to certain CPU (or other)
facilities can be blocked by the hypervisor. PAPR provides ways to
advertise in the device tree whether or not those features are available to
the guest.
In some places we automatically determine whether to make a feature
available based on whether our host can support it, in most cases this is
based on limitations in the available KVM implementation.
Although we correctly advertise this to the guest, it means that host
factors might make changes to the guest visible environment which is bad:
as well as generaly reducing reproducibility, it means that a migration
between different host environments can easily go bad.
We've mostly gotten away with it because the environments considered mature
enough to be well supported (basically, KVM on POWER8) have had consistent
feature availability. But, it's still not right and some limitations on
POWER9 is going to make it more of an issue in future.
This introduces an infrastructure for defining "sPAPR capabilities". These
are set by default based on the machine version, masked by the capabilities
of the chosen cpu, but can be overriden with machine properties.
The intention is at reset time we verify that the requested capabilities
can be supported on the host (considering TCG, KVM and/or host cpu
limitations). If not we simply fail, rather than silently modifying the
advertised featureset to the guest.
This does mean that certain configurations that "worked" may now fail, but
such configurations were already more subtly broken.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
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