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

[Anthony Liguori]

On 08/04/2010 08:34 AM, Gleb Natapov wrote:
> On Wed, Aug 04, 2010 at 08:15:04AM -0500, Anthony Liguori wrote:
>    
> > On 08/04/2010 08:07 AM, Gleb Natapov wrote:
> >      
> > > On Wed, Aug 04, 2010 at 08:04:09AM -0500, Anthony Liguori wrote:
> > >        
> > > > On 08/04/2010 03:17 AM, Avi Kivity wrote:
> > > >          
> > > > > For playing games, there are three options:
> > > > > - existing fwcfg
> > > > > - fwcfg+dma
> > > > > - put roms in 4GB-2MB (or whatever we decide the flash size is)
> > > > > and have the BIOS copy them
> > > > >
> > > > > Existing fwcfg is the least amount of work and probably
> > > > > satisfactory for isapc.  fwcfg+dma is IMO going off a tangent.
> > > > > High memory flash is the most hardware-like solution, pretty easy
> > > > >            
> > > > >from a qemu point of view but requires more work.
> > > >
> > > > The only trouble I see is that high memory isn't always available.
> > > > If it's a 32-bit PC and you've exhausted RAM space, then you're only
> > > > left with the PCI hole and it's not clear to me if you can really
> > > > pull out 100mb of space there as an option ROM without breaking
> > > > something.
> > > >
> > > >          
> > > We can map it on demand. Guest tells qemu to map rom "A" to address X by
> > > writing into some io port. Guest copies rom. Guest tells qemu to unmap
> > > it. Better then DMA interface IMHO.
> > >        
> > That's what I thought too, but in a 32-bit guest using ~3.5GB of
> > RAM, where can you safely get 100MB of memory to full map the ROM?
> > If you're going to map chunks at a time, you are basically doing
> > DMA.
> >
> >      
> This is not like DMA event if done in chunks and chunks can be pretty
> big. The code that dials with copying may temporary unmap some pci
> devices to have more space there.
>    

That's a bit complicated because SeaBIOS is managing the PCI devices 
whereas the kernel code is running as an option rom.  I don't know the 
BIOS PCI interfaces well so I don't know how doable this is.

Maybe we're just being too fancy here.

We could rewrite -kernel/-append/-initrd to just generate a floppy image 
in RAM, and just boot from floppy.

Regards,

Anthony Liguori

>    
> > And what's the upper limit on ROM size that we impose?  100MB is
> > already at the ridiculously large size.
> >
> >      
> Agree. We have two solutions:
> 1. Avoid the problem
> 2. Fix the problem.
>
> Both are fine with me and I prefer 1, but if we are going with 2 I
> prefer something sane.
>
> --
>                         Gleb.
>