Re: [Qemu-devel] [PATCH v2 1/2] memory: Fix old portio word accesses

[Avi Kivity]

On 09/19/2011 03:55 PM, Jan Kiszka wrote:
> >
> >  The trick of having a way to register N callbacks with one shot is worth
> >  growing.  Ideally each register in a BAR would have a callback and we'd
> >  do something like
> >
> >      MemoryRegionOps mydev_ops = {
> >          .registers = {
> >               { MYDEV_REG_x, 4, 4, mydev_reg_x_read, mydev_reg_x_write, },
> >               ...
> >           },
> >      }
> >
> >  with hints to the core like "this register sits at this offset, use it
> >  for reads instead of a callback", or, "this is a read-only register".
>
> This has pros and cons. If you have n registers to dispatch, you then
> have to write n function prologues and maybe epilogues instead of just
> one. Specifically if the register access is trivial, that could case
> quite some LoC blowup on the device side.
>
> What may have a better ratio are generic register get/set handlers.

With C++ pointers-to-members and pointers-to-member-functions, you 
actually get some nice representation:

class MyDev {

    void reg_1_read(...) { return some_computation(); }
    void reg_1_write(...) { do_something(); }

    uint32_t reg_2;
    void reg_2_write(...) { reg_2 = value; do_something(); }

    uint64_t reg_3;

    static const Register registers[] = {
          Register(REG_1, &MyDev::reg_1_read, &MyDev::reg_1_write),
          Register(REG_2, &MyDev::reg_2, &MyDev::reg_2_write),
          Register(REG_1, &MyDev::reg_3),
    };
};

... and the Register class generates the appropriate accessors.  We can 
emulate some of this with macros, but the conversion from opaque to the 
actual type will always be ugly.


--
error compiling committee.c: too many arguments to function