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

[Avi Kivity <avi@redhat.com>]

On 02/07/2012 05:23 PM, 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.
>
> But not all architectures offer non-word sized atomic instructions so
> it gets fairly nasty in practice.
>

I doubt that any guest accesses IDE registers from two threads in
parallel.  The guest will have some lock, so we could have a lock as
well and be assured that there will never be contention.

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