Re: [Qemu-devel] using mmap?

[Paul Brook <paul@codesourcery.com>]

> Does anyone know the reason for the removal of the mmap()? I have used 
> a benchmarking tool (I think it was 3D Mark05 or 3D Mark06) and the
> memory access in the guest WinXP was slooooooow.  Does anyone have any
> insight on making the hardware MMU function for linux-x86 host to WinXP
> 32-bit guest, even partially? 

As I already said, the code required a modified guest OS. i.e. it wouldn't 
work with windows guests anyway.

It was removed because it provided very little benefit, and wasn't worth the 
effort to keep it working.  Its uses were very specialised, and IMHO better 
covered by other products like UML or xen.

I'm also doubtful how much benefit it gave in practice. I'm sure it would be 
good for synthetic CPU benchmarks. However using mmap significantly increases 
the overhead of context switches/tlb misses.

To get good overall performance I suspect you're going to need closer 
cooperation with the kernel than mmap gives you. If you really want to make 
cross-emulation go fast I suggest working with the xen and/or kvm people to 
integrate qemu dynamic translation into those products. In theory I'd guess 
you should be able to plug it in as an alternative to the HVM code. I've no 
idea how close that is to being practical.

> Wouldn't this be a *significant* 
> performance enhancement for this setup which I'm sure is a common one?
> Maybe this can be implemented for regular processes on the guest and
> only use the softmmu for the kernel?  Would someone point me in the
> right direction for technical information?  I have had to switch to
> vmware free player until Qemu+KQEMU reaches a point of similar
> performance, but I would really rather see Qemu advance further.

If you're using an accelerator (eg. kqemu or kvm) this is all irelevant as 
most code isn't run by qemu, it's virtualized by the accelerator. qemu just 
does the IO emulation.

Paul