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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: Wed, 04 Aug 2010 12:36:18 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100713 Lightning/1.0b1 Thunderbird/3.0.6

On 08/04/2010 12:31 PM, Alexander Graf wrote:
On 04.08.2010, at 19:26, Anthony Liguori wrote:

On 08/04/2010 11:45 AM, Alexander Graf wrote:
Frankly, I partially agreed to your point when we were talking about 300ms vs. 
2 seconds. Now that we're talking 8 seconds that whole point is moot. We chose 
the wrong interface to transfer kernel+initrd data into the guest.

Now the question is how to fix that. I would veto against anything normally 
guest-OS-visible. By occupying the floppy, you lose a floppy drive in the 
guest. By occupying a disk, you see an unwanted disk in the guest.

Introduce a new virtio device type (say, id 6).  Teach SeaBIOS that 6 is 
exactly like virtio-blk (id 2).  Make it clear that id 6 is only to be used by 
firmware and that normal guest drivers should not be written for id 6.
Why not make id 6 be a fw_cfg virtio interface?

Because that's a ton more work and we need fw_cfg to be available before PCI is. IOW, fw_cfg cannot be a PCI interface.


Anthony Liguori

  That way we'd stay 100% compatible to everything we have and also get a fast 
path for reading big chunks of data from fw_cfg. All we'd need is a command to 
set the 'file' we're in.

Even better yet, why not use virtio-9p and expose all of fw_cfg as files? Then 
implement a simple virtio-9p client in SeaBIOS and maybe even get direct 
kernel/initrd boot from a real 9p system ;).


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