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

From: Anthony Liguori
Subject: Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?
Date: Tue, 03 Aug 2010 14:38:21 -0500
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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.


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.


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.

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