QEMU implements the "Shared Processor LPAR" (SPLPAR) option, which allows
the hypervisor to time-slice a physical processor into multiple virtual
processor. The intent is to allow more guests to run, and to optimize
processor utilization.
The guest OS can cede idle VCPUs, so that their processing capacity may
be used by other VCPUs, with the H_CEDE hcall. The guest OS can also
optimize spinlocks, by confering the time-slice of a spinning VCPU to the
spinlock holder if it's currently notrunning, with the H_CONFER hcall.
Both hcalls depend on a "Virtual Processor Area" (VPA) to be registered
by the guest OS, generally during early boot. Other per-VCPU areas can
be registered: the "SLB Shadow Buffer" which allows a more efficient
dispatching of VCPUs, and the "Dispatch Trace Log Buffer" (DTL) which
is used to compute time stolen by the hypervisor. Both DTL and SLB Shadow
areas depend on the VPA to be registered.
The VPA/SLB Shadow/DTL are state that QEMU should migrate, but this doesn't
happen, for no apparent reason other than it was just never coded. This
causes the features listed above to stop working after migration, and it
breaks the logic of the H_REGISTER_VPA hcall in the destination.
The VPA is set at the guest request, ie, we don't have to migrate
it before the guest has actually set it. This patch hence adds an
"spapr_cpu/vpa" subsection to the recently introduced per-CPU machine
data migration stream.
Since DTL and SLB Shadow are optional and both depend on VPA, they get
their own subsections "spapr_cpu/vpa/slb_shadow" and "spapr_cpu/vpa/dtl"
hanging from the "spapr_cpu/vpa" subsection.
Note that this won't break migration to older QEMUs. Is is already handled
by only registering the vmstate handler for per-CPU data with newer machine
types.
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
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