• Paul Mackerras's avatar
    KVM: PPC: Book3S HV: Implement timebase offset for guests · 93b0f4dc
    Paul Mackerras authored
    This allows guests to have a different timebase origin from the host.
    This is needed for migration, where a guest can migrate from one host
    to another and the two hosts might have a different timebase origin.
    However, the timebase seen by the guest must not go backwards, and
    should go forwards only by a small amount corresponding to the time
    taken for the migration.
    
    Therefore this provides a new per-vcpu value accessed via the one_reg
    interface using the new KVM_REG_PPC_TB_OFFSET identifier.  This value
    defaults to 0 and is not modified by KVM.  On entering the guest, this
    value is added onto the timebase, and on exiting the guest, it is
    subtracted from the timebase.
    
    This is only supported for recent POWER hardware which has the TBU40
    (timebase upper 40 bits) register.  Writing to the TBU40 register only
    alters the upper 40 bits of the timebase, leaving the lower 24 bits
    unchanged.  This provides a way to modify the timebase for guest
    migration without disturbing the synchronization of the timebase
    registers across CPU cores.  The kernel rounds up the value given
    to a multiple of 2^24.
    
    Timebase values stored in KVM structures (struct kvm_vcpu, struct
    kvmppc_vcore, etc.) are stored as host timebase values.  The timebase
    values in the dispatch trace log need to be guest timebase values,
    however, since that is read directly by the guest.  This moves the
    setting of vcpu->arch.dec_expires on guest exit to a point after we
    have restored the host timebase so that vcpu->arch.dec_expires is a
    host timebase value.
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarAlexander Graf <agraf@suse.de>
    93b0f4dc
reg.h 51.1 KB