• Paul Mackerras's avatar
    KVM: PPC: Allow book3s_hv guests to use SMT processor modes · 371fefd6
    Paul Mackerras authored
    This lifts the restriction that book3s_hv guests can only run one
    hardware thread per core, and allows them to use up to 4 threads
    per core on POWER7.  The host still has to run single-threaded.
    
    This capability is advertised to qemu through a new KVM_CAP_PPC_SMT
    capability.  The return value of the ioctl querying this capability
    is the number of vcpus per virtual CPU core (vcore), currently 4.
    
    To use this, the host kernel should be booted with all threads
    active, and then all the secondary threads should be offlined.
    This will put the secondary threads into nap mode.  KVM will then
    wake them from nap mode and use them for running guest code (while
    they are still offline).  To wake the secondary threads, we send
    them an IPI using a new xics_wake_cpu() function, implemented in
    arch/powerpc/sysdev/xics/icp-native.c.  In other words, at this stage
    we assume that the platform has a XICS interrupt controller and
    we are using icp-native.c to drive it.  Since the woken thread will
    need to acknowledge and clear the IPI, we also export the base
    physical address of the XICS registers using kvmppc_set_xics_phys()
    for use in the low-level KVM book3s code.
    
    When a vcpu is created, it is assigned to a virtual CPU core.
    The vcore number is obtained by dividing the vcpu number by the
    number of threads per core in the host.  This number is exported
    to userspace via the KVM_CAP_PPC_SMT capability.  If qemu wishes
    to run the guest in single-threaded mode, it should make all vcpu
    numbers be multiples of the number of threads per core.
    
    We distinguish three states of a vcpu: runnable (i.e., ready to execute
    the guest), blocked (that is, idle), and busy in host.  We currently
    implement a policy that the vcore can run only when all its threads
    are runnable or blocked.  This way, if a vcpu needs to execute elsewhere
    in the kernel or in qemu, it can do so without being starved of CPU
    by the other vcpus.
    
    When a vcore starts to run, it executes in the context of one of the
    vcpu threads.  The other vcpu threads all go to sleep and stay asleep
    until something happens requiring the vcpu thread to return to qemu,
    or to wake up to run the vcore (this can happen when another vcpu
    thread goes from busy in host state to blocked).
    
    It can happen that a vcpu goes from blocked to runnable state (e.g.
    because of an interrupt), and the vcore it belongs to is already
    running.  In that case it can start to run immediately as long as
    the none of the vcpus in the vcore have started to exit the guest.
    We send the next free thread in the vcore an IPI to get it to start
    to execute the guest.  It synchronizes with the other threads via
    the vcore->entry_exit_count field to make sure that it doesn't go
    into the guest if the other vcpus are exiting by the time that it
    is ready to actually enter the guest.
    
    Note that there is no fixed relationship between the hardware thread
    number and the vcpu number.  Hardware threads are assigned to vcpus
    as they become runnable, so we will always use the lower-numbered
    hardware threads in preference to higher-numbered threads if not all
    the vcpus in the vcore are runnable, regardless of which vcpus are
    runnable.
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarAlexander Graf <agraf@suse.de>
    371fefd6
icp-native.c 6.6 KB