Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?

[Anthony Liguori]

On 08/03/2010 02:24 PM, Avi Kivity wrote:
>  On 08/03/2010 10:15 PM, Anthony Liguori wrote:
> > fw_cfg has to be available pretty early on so relying on a PCI device 
> > isn't reasonable.  Having dual interfaces seems wasteful.
>
> Agree.
>
> > We're already doing bulk data transfer over fw_cfg as we need to do 
> > it to transfer roms and potentially a boot splash. 
>
> Why do we need to transfer roms?  These are devices on the memory bus 
> or pci bus, it just needs to be there at the right address.

Not quite.  The BIOS owns the option ROM space.  The way it works on 
bare metal is that the PCI ROM BAR gets mapped to some location in 
physical memory by the BIOS, the BIOS executes the initialization 
vector, and after initialization, the ROM will reorganize itself into 
something smaller.  It's nice and clean.

But ISA is not nearly as clean.  Ultimately, to make this mix work in a 
reasonable way, we have to provide a side channel interface to SeaBIOS 
such that we can deliver ROMs outside of PCI and still let SeaBIOS 
decide how ROMs get organized.

It's additionally complicated by the fact that we didn't support PCI ROM 
BAR until recently so to maintain compatibility with -M older, we have 
to use a side channel to lay out option roms.

Regards,

Anthony Liguori

>   Boot splash should just be another rom as it would be on a real system.
>
> > Even outside of loading an initrd, the performance is going to start 
> > to matter with a large number of devices.
>
> I don't really see why.