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    SubjectRe: TSCs are a no-no on i386
    On Thu, 31 Jul 2003, Jamie Lokier wrote:

    > Willy Tarreau wrote:
    > > The other problem lies with the lock :
    > > When a 486 executes "LOCK ; CMPXCHG", it locks the bus during the whole cmpxchg
    > > instruction. If a 386 executes the same code, it will get an exception which
    > > will be caught by the emulator. I don't see how we can do such an atomic
    > > operation while holding a lock. At best, we would think about a global memory
    > > based shared lock during the operation (eg: int bus_lock;), but it's not
    > > implemented at the moment, and will only be compatible with processors sharing
    > > the same code. Add-on processors, such as co-processors, transputer cards, or
    > > DSPs, will know nothing about such a lock emulation. And it would result in
    > > even poorer performance of course !
    > Of course this is not a problem when "lock;cmpxchg" is used only for thread
    > synchronisation on uniprocessor 386s... The lock prefix is irrelevant then.
    > Perhaps the emulation should refuse to pretend to work on an SMP 386 :)
    > -- Jamie
    > -

    You can use the lock instruction ahead of any 386 instruction
    without creating an exception. When relevent, it locks the whole
    bus. When not, it's just a no-op. The trap on the lock instruction
    came with the '486. With the '486, if the instruction doesn't
    modify memory, then the lock prefix is invalid and will generate
    an invalid-opcode exception.

    It is not correct to use a lock instruction in front of every
    op-code of course, and it might not have been tested for all
    corner cases, but generally it's harmless on a '386.

    The bad op-code for the i386 is cmpxchg. This is what triggers
    the trap. This can be emulated, although the emulation is
    not SMP compatible. You worry about this when somebody makes
    a dual '386 machine ;^). Also, the best performing emulation
    for any op-codes should be done within the kernel. That way,
    the invalid-opcode trap works just like the math emulator. You
    don't need the overhead of calling a user-mode handler. If
    this is emulation is implimented, then one should also emulate
    BSWAP and XADD. This makes '486 code compatible with '386

    Dick Johnson
    Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
    Note 96.31% of all statistics are fiction.

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