Re: [Qemu-devel] DMA in Windows 2000/XP/2003

[André Braga]

Fortunately, it does make a difference.

PIO is polling-base, whereas DMA is, lacking a better term (excuse my
English), transaction-based. Since no CPU arbitration is needed, quite
a few optimizations can be done because of this, like real, large
block transfers. And if you happen to search the list archives, people
have already reported significant gains when using these patches, so
even if I'm talking bullocks empirical results do exist :)


Cheers,
A.


-- 
"I invented the term Object-Oriented, and I can tell you I did not
have C++ in mind"
-Alan Kay


2005/8/24, Jan Marten Simons <address@hidden>:
> Víctor Córcoles López wrote:
> 
> > Hello developers. My English is not good.
> >
> > I see that DMA in Hard Disks in guest OS Windows 2000/XP/2003 is not
> > avalaible, it run in PIO mode.
> >
> > How can activate UDMA mode for hard disk ?
> >
> I don't think you'd get any advantage of activating DMA inside the qemu
> guest-OS, as qemu has to proxy all rw-access into a file anyway, unless
> you pass an actual device/partition to qemu. Besides qemu is emulating
> the complete pci bus, which cannot use features of actual hardware in
> the machine (e.g. your hdd or features your chipset provide). I guess it
> would be possible to emulate uDMA inside the vm, too, but it's likely to
> even slowdown the emulation.
> 
> So long,
> Jan