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

[Alexander Graf]

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