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

[Gleb Natapov <gleb@redhat.com>]

On Wed, Aug 04, 2010 at 08:52:44AM -0500, Anthony Liguori wrote:
> 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.
> 
Unmapping device and mapping it at the same place is easy. Enumerating
pci devices from multiboot.bin looks like unneeded churn though.

> 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.
> 
May be. Can floppy be 100M?

--
			Gleb.