• Cyril Bur's avatar
    powerpc: tm: Always use fp_state and vr_state to store live registers · dc310669
    Cyril Bur authored
    There is currently an inconsistency as to how the entire CPU register
    state is saved and restored when a thread uses transactional memory
    (TM).
    
    Using transactional memory results in the CPU having duplicated
    (almost) all of its register state. This duplication results in a set
    of registers which can be considered 'live', those being currently
    modified by the instructions being executed and another set that is
    frozen at a point in time.
    
    On context switch, both sets of state have to be saved and (later)
    restored. These two states are often called a variety of different
    things. Common terms for the state which only exists after the CPU has
    entered a transaction (performed a TBEGIN instruction) in hardware are
    'transactional' or 'speculative'.
    
    Between a TBEGIN and a TEND or TABORT (or an event that causes the
    hardware to abort), regardless of the use of TSUSPEND the
    transactional state can be referred to as the live state.
    
    The second state is often to referred to as the 'checkpointed' state
    and is a duplication of the live state when the TBEGIN instruction is
    executed. This state is kept in the hardware and will be rolled back
    to on transaction failure.
    
    Currently all the registers stored in pt_regs are ALWAYS the live
    registers, that is, when a thread has transactional registers their
    values are stored in pt_regs and the checkpointed state is in
    ckpt_regs. A strange opposite is true for fp_state/vr_state. When a
    thread is non transactional fp_state/vr_state holds the live
    registers. When a thread has initiated a transaction fp_state/vr_state
    holds the checkpointed state and transact_fp/transact_vr become the
    structure which holds the live state (at this point it is a
    transactional state).
    
    This method creates confusion as to where the live state is, in some
    circumstances it requires extra work to determine where to put the
    live state and prevents the use of common functions designed (probably
    before TM) to save the live state.
    
    With this patch pt_regs, fp_state and vr_state all represent the
    same thing and the other structures [pending rename] are for
    checkpointed state.
    Acked-by: default avatarSimon Guo <wei.guo.simon@gmail.com>
    Signed-off-by: default avatarCyril Bur <cyrilbur@gmail.com>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    dc310669
process.c 49.3 KB