SharedUserData

[Laurent Pinchart]

Hi everybody,

I need to implement the SharedUserData, which is a page of memory 
located at (KERNEL_BASE - 0x10000) shared between user space (read-only 
access) and kernel space.

Some information in that page are very accurate (system time in 100ns 
units for instance), so I don't think that it would be possible to 
create a shared page at (KERNEL_BASE - 0x10000) and refresh it fast enough.

Please correct me if I'm wrong, but I think that the way to go would be 
to create a copy of that page in memory and initialize it when Wine 
starts (some information never change, like the system root or version 
number). When a user space program would try to access that page, the 
page fault exception handler would return the appropriate data.

I'm not very familiar with Wine internals, so I might be wrong, or 
forget some important issues. Comments are more than welcome :-)

I'd also appreciate if someone could give me some hints about the 
implementation (where I should look for the page fault exception 
handler, where I should initialize that shared data, where I can find 
the needed information to initialize those data, ...).

Thanks in advance.

Laurent Pinchart

[David Laight]

On Wed, Apr 03, 2002 at 09:49:07AM +0200, Laurent Pinchart wrote:
> Hi everybody,
>
> I need to implement the SharedUserData, which is a page of memory 
> located at (KERNEL_BASE - 0x10000) shared between user space (read-only 
> access) and kernel space.

I don't know how to get the 'page fault' trap for a 'valid' user
page on all systems that wine might run on.  Certainly the assumption
that (KERNEL_BASE - 0x10000) is available is probably false...

However you don't actually need a valid user address, a kernel
address (ie one that is always invalid) will do.
Catching SIGSEGV might be enough - but I'm not totally
certain how you can get the value back into the main
process context (emulate the instruction and modify the
saved program counter ????)

David

[Laurent Pinchart]

> > I need to implement the SharedUserData, which is a page of memory 
> > located at (KERNEL_BASE - 0x10000) shared between user space (read-only 
> > access) and kernel space.
>
> I don't know how to get the 'page fault' trap for a 'valid' user
> page on all systems that wine might run on.  Certainly the assumption
> that (KERNEL_BASE - 0x10000) is available is probably false...
>
> However you don't actually need a valid user address, a kernel
> address (ie one that is always invalid) will do.

I'm not sure to understand what you mean. The user program will try to 
access KERNEL_BASE - 0x10000 (which is 0x7ffe0000 on IA-32). I need that 
to trigger a segfault.
> Catching SIGSEGV might be enough - but I'm not totally
> certain how you can get the value back into the main
> process context (emulate the instruction and modify the
> saved program counter ????)

That's what I thought at first. I could also try to set a hardware 
breakpoint for read/write accesses at that address, but that's highly 
not portable, and won't work nicely with a debugger. If someone has a 
better idea I'll be glad to hear it.

Laurent Pinchart