[Qemu-devel] [patch] IDE busmaster DMA support

[John Coiner]

Hi,

Here's another patch to enable IDE DMA:

  http://people.brandeis.edu/~jcoiner/qemu_idedma/qemu_dma_patch.html

This patch, like Juergen Lock's earlier patch from which it is derived, 
replaces the virtual PIIX3 IDE controller with a PIIX4.

It applies cleanly to 0.7.2.

This patch extends the Windows 2000 install hack to work for DMA also -- 
I installed and ran Windows 2000 with DMA enabled.

It'd be cool if QEMU supported nonblocking disk access -- if it would 
let the guest resume running currently, while waiting for the disk to 
finish seeking. This patch is a first step toward concurrent IO, which 
would probably have to use busmastering DMA transfers to be efficient, 
if it's going to be efficient at all.

Do people have ideas about how to implement concurrent IO? I have a 
couple hazy ideas...

Can anyone shed light on the potential performance to be realized? For 
example, how long does a context switch take in the guest? If it takes 
10ms (one disk seek) then there's probably no benefit to concurrent disk 
IO. If a context switch takes 1ms, then the guest has opportunity to 
make forward progress while awaiting an IDE interrupt.

Regards, happy hacking,

John