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Re: [Qemu-devel] [RFC] Next gen kvm api

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On 02/07/2012 07:40 AM, Alexander Graf wrote:

Why? For the HPET timer register for example, we could have a simple MMIO hook that says

   on_read:
     return read_current_time() - shared_page.offset;
   on_write:
     handle_in_user_space();

For IDE, it would be as simple as

   register_pio_hook_ptr_r(PIO_IDE, SIZE_BYTE,&s->cmd[0]);
   for (i = 1; i<  7; i++) {
     register_pio_hook_ptr_r(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
     register_pio_hook_ptr_w(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
   }

You can't easily serialize updates to that address with the kernel since two threads are likely going to be accessing it at the same time. That either means an expensive sync operation or a reliance on atomic instructions.

But not all architectures offer non-word sized atomic instructions so it gets fairly nasty in practice.

Regards,

Anthony Liguori
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