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Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2

From: Gleb Natapov
Subject: Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?
Date: Wed, 4 Aug 2010 16:34:01 +0300

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.

> 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.


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