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

[Anthony Liguori]

On 08/03/2010 02:41 PM, Avi Kivity wrote:
>  On 08/03/2010 10:38 PM, Anthony Liguori wrote:
> > > 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. 
>
> So far so good.
>
> > 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.
>
> I don't follow.  Why do we need this side channel?  What would a real 
> ISA machine do?

It depends on the ISA machine.  In the worst case, there's a DIP switch 
on the card and if you've got a conflict between two cards, you start 
flipping DIP switches.  It's pure awesomeness.  No, I don't want to 
emulate DIP switches :-)

>   Are there actually enough ISA devices for there to be a problem?

No, but -M older has the same problem.

> > 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.
>
> Again I don't follow.  We can just lay out the ROMs in memory like we 
> did in the past?

Because only one component can own the option ROM space.  Either that's 
SeaBIOS and we need a side channel or it's QEMU and we can't use PMM.

I guess that's the real issue here.  Previously we used etherboot which 
was well under 32k.  We only loaded roms we needed.  Now we use gPXE 
which is much bigger and if you don't use PMM, then you run out of 
option rom space very quickly.

Previously, we loaded option ROMs on demand when a user used -boot n but 
that was a giant hack and wasn't like bare metal at all.  It involved 
x86-isms in vl.c.  Now we always load ROMs so PMM is very important.

Regards,

Anthony Liguori