Setting the CPU affinity of QEMU threads is a bit problematic, because
QEMU doesn't always have permissions to set the CPU affinity itself,
for example, with seccomp after initialized by QEMU:
    -sandbox enable=on,resourcecontrol=deny

General information about CPU affinities can be found in the man page of
taskset:
    CPU affinity is a scheduler property that "bonds" a process to a given
    set of CPUs on the system. The Linux scheduler will honor the given CPU
    affinity and the process will not run on any other CPUs.

While upper layers are already aware of how to handle CPU affinities for
long-lived threads like iothreads or vcpu threads, especially short-lived
threads, as used for memory-backend preallocation, are more involved to
handle. These threads are created on demand and upper layers are not even
able to identify and configure them.

Introduce the concept of a ThreadContext, that is essentially a thread
used for creating new threads. All threads created via that context
thread inherit the configured CPU affinity. Consequently, it's
sufficient to create a ThreadContext and configure it once, and have all
threads created via that ThreadContext inherit the same CPU affinity.

The CPU affinity of a ThreadContext can be configured two ways:

(1) Obtaining the thread id via the "thread-id" property and setting the
    CPU affinity manually (e.g., via taskset).

(2) Setting the "cpu-affinity" property and letting QEMU try set the
    CPU affinity itself. This will fail if QEMU doesn't have permissions
    to do so anymore after seccomp was initialized.

A simple QEMU example to set the CPU affinity to host CPU 0,1,6,7 would be:
    qemu-system-x86_64 -S \
      -object thread-context,id=tc1,cpu-affinity=0-1,cpu-affinity=6-7

And we can query it via HMP/QMP:
    (qemu) qom-get tc1 cpu-affinity
    [
        0,
        1,
        6,
        7
    ]

But note that due to dynamic library loading this example will not work
before we actually make use of thread_context_create_thread() in QEMU
code, because the type will otherwise not get registered. We'll wire
this up next to make it work.

In general, the interface behaves like pthread_setaffinity_np(): host
CPU numbers that are currently not available are ignored; only host CPU
numbers that are impossible with the current kernel will fail. If the
list of host CPU numbers does not include a single CPU that is
available, setting the CPU affinity will fail.

A ThreadContext can be reused, simply by reconfiguring the CPU affinity.
Note that the CPU affinity of previously created threads will not get
adjusted.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221014134720.168738-4-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>