Are linker scripts needed?

Are linker scripts needed?

[Arun Sharma]

In alt.bk.changesets.1.1508 Keir wrote:

> Some more ioemu config hardcoding. Our 'target arch' is always i386.
> Do we really need linker scripts??
> Signed-off-by: Keir Fraser <keir@xensource.com>

Yes, we do need linker scripts to support VMX guests with large memory 
(> 256MB). Without the linker scripts, we found instabilities with a 
512MB guest.

The reason for this is, qemu is mapping all of guest physical in it's 
address space. The linker script moves all of the qemu text/data/stack 
to the upper part of the user virtual to leave a large enough hole for 
mapping guest physical using xc_map_foreign.

Our intention is to use qemu for i386, x86-64 and ia64. The 
"target-i386-dm" is a misnomer and inherited from upstream. I should 
probably kill the concept of a "target architecture" since ioemu is cpu 
architecture independent right now.

	-Arun

Re: Are linker scripts needed?

[Keir Fraser]

On 23 May 2005, at 07:00, Arun Sharma wrote:

> Yes, we do need linker scripts to support VMX guests with large memory 
> (> 256MB). Without the linker scripts, we found instabilities with a 
> 512MB guest.
>
> The reason for this is, qemu is mapping all of guest physical in it's 
> address space. The linker script moves all of the qemu text/data/stack 
> to the upper part of the user virtual to leave a large enough hole for 
> mapping guest physical using xc_map_foreign.

I suppose it improves performance and simplifies the data path, but it 
should probably dynamically map the vmx guest's pages for I/O on 
demand. After all, this approach isn't going to work for bigmem domains 
on i386 (particularly once PAE is working). So it seems a bad idea to 
come to rely on it any more than as a bit of a hack to get you up and 
running.

Best of all (imo) would be for it to act as a 'driver in the middle': 
connecting the fully virt guest through to an ordinary backend driver, 
and not itself touching I/O buffers at all. Perhaps ultimately we can 
push the qemu drivers into the vmx container itself, perhaps in 
'virtual smm mode', so that from outside the container it looks just 
like any other paravirt I/O channel.

> Our intention is to use qemu for i386, x86-64 and ia64. The 
> "target-i386-dm" is a misnomer and inherited from upstream. I should 
> probably kill the concept of a "target architecture" since ioemu is 
> cpu architecture independent right now.

Sounds good.

  -- Keir

Re: Re: Are linker scripts needed?

[Keir Fraser]

On 23 May 2005, at 09:02, Keir Fraser wrote:

>
>> Our intention is to use qemu for i386, x86-64 and ia64. The 
>> "target-i386-dm" is a misnomer and inherited from upstream. I should 
>> probably kill the concept of a "target architecture" since ioemu is 
>> cpu architecture independent right now.
>
> Sounds good.

In fact, I think it would be good to kill the configure script and have 
a clean up of the Makefiles -- it's really hard to see what's going on 
in them right now, and we aren't going to use all that 
flexibility/genericity anyway. I reckon the Makefiles should end up a 
good deal shorter and clearer.

  -- Keir

Re: Re: Are linker scripts needed?

[Arun Sharma]

Keir Fraser wrote:
> 
> In fact, I think it would be good to kill the configure script and have 
> a clean up of the Makefiles -- it's really hard to see what's going on 
> in them right now, and we aren't going to use all that 
> flexibility/genericity anyway. I reckon the Makefiles should end up a 
> good deal shorter and clearer.

Agree that configure isn't adding much value today. The only useful 
thing it's doing is to detect if the build env has libvncserver and 
disabling VNC support if it doesn't have it.

Will send a makefile patch as soon as I have one...

	-Arun