On 07.02.2012, at 16:23, Anthony Liguori wrote:
> 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.
Yes. Essentially we want a mutex for them.
> But not all architectures offer non-word sized atomic instructions so it gets
> fairly nasty in practice.
Well, we can always require fields to be word sized.
Alex
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