Re: [kvm-devel] [RFC PATCH 1/5] lguest: mmap backing file

[Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>]

On Friday 21 March 2008 01:04:17 Anthony Liguori wrote:
> Rusty Russell wrote:
> > From: Paul TBBle Hampson <Paul.Hampson-vM6MUUi4OUAAvxtiuMwx3w@public.gmane.org>
> >
> > This creates a file in $HOME/.lguest/ to directly back the RAM and DMA
> > memory mappings created by map_zeroed_pages.
>
> I created a test program recently that measured the latency of a
> reads/writes to an mmap() file in /dev/shm and in a normal filesystem.
> Even after unlinking the underlying file, the write latency was much
> better with a mmap()'d file in /dev/shm.

How odd!  Do you have any idea why?

> /dev/shm is not really for general use.  I think we'll want to have our
> own tmpfs mount that we use to create VM images.

If we're going to mod the kernel, how about a "mmap this part of their address 
space" and having the kernel keep the mappings in sync.  But I think that if 
we want to get speed, we should probably be doing the copy between address 
spaces in-kernel so we can do lightweight exits.

> I also prefer to use a 
> unix socket for communication, unlink the file immediately after open,
> and then pass the fd via SCM_RIGHTS to the other process.

Yeah, I shied away from that because cred passing kills whole litters of 
puppies.  It makes for better encapsulation tho, so I'd do it that way in a 
serious implementation.

Cheers,
Rusty.