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Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack int


From: Anthony Liguori
Subject: Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends
Date: Tue, 11 Oct 2011 08:19:14 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.21) Gecko/20110831 Lightning/1.0b2 Thunderbird/3.1.13

On 10/11/2011 08:14 AM, Alexander Graf wrote:
And I don't see the point why we would have to shoot yet another hole into the 
guest just because we're too unwilling to make an interface that's perfectly 
valid horribly slow.

rep/ins is exactly like dma+wait for this use case: provide an address, get a 
memory image in return.  There's no need to add another interface, we should 
just optimize the existing one.

Whatever we do, the interface will never be as fast as DMA. We will always have 
to do sanity / permission checks for every IO operation, can batch up only so 
many IO requests and in QEMU again have to call our callbacks in a loop.

rep/ins is effectively equivalent to DMA except in how it's handled within QEMU.

No, DMA has a lot bigger granularities in kvm/user interaction. We can easily 
DMA a 50MB region with a single kvm/user exit. For PIO we can at most do page 
granularity.

So make a proper PCI device for kernel loading. It's a much more natural approach and let's use alias -kernel/-initrd/-append to -device kernel-pci,kernel=PATH,initrd=PATH

Regards,

Anthony Liguori


Alex






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