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