Re: [Qemu-devel] [PATCH 3/3] disk: don't read from disk until the guest starts

[Avi Kivity <avi@redhat.com>]

  On 09/12/2010 05:29 PM, Anthony Liguori wrote:
> On 09/12/2010 08:26 AM, Avi Kivity wrote:
>>  On 09/12/2010 03:08 PM, Anthony Liguori wrote:
>>>> This can cause a disk read, no?  Shouldn't it be made asynchronous?
>>>
>>>
>>> Yes, it should.  I'm not sure there's an easy way to make it 
>>> asynchronous though not because of the block layer but because of 
>>> how these functions are called.
>>
>> Sorry to harp on the subject, but that's the standard problem with 
>> state machines.  Every time you want to do a blocking operation in a 
>> function, you have to put all its locals in some structure, split the 
>> function into two, do some scheduling, etc.
>
> We can't block the VCPU thread for arbitrarily long periods of time.  
> If we get a PIO operation requesting information about geometry, we 
> can't wait for a disk read in order to satisfy that request.
>
> We need to kick off the I/O operations in the background such that the 
> data is available before the PIO operation happens.  This isn't SM vs. 
> threads at all, this is simply about the fact that we can't do block 
> I/O during a PIO operation.

Isn't this an identify command, where the guest can only read the data 
after the interface indicates it is ready (or posts an interrupt)?  I 
thought the guest interface was already async.

The code appears to support this:

         switch(val) {
         case WIN_IDENTIFY:
             if (s->bs && s->drive_kind != IDE_CD) {
                 if (s->drive_kind != IDE_CFATA)
                     ide_identify(s);
                 else
                     ide_cfata_identify(s);
                 s->status = READY_STAT | SEEK_STAT;
                 ide_transfer_start(s, s->io_buffer, 512, 
ide_transfer_stop);
             } else {
                 if (s->drive_kind == IDE_CD) {
                     ide_set_signature(s);
                 }
                 ide_abort_command(s);
             }
             ide_set_irq(s->bus);
             break;

but I may be misinterpreting it.  If I'm right, all we need to do is 
push this whole block to a thread.

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