[vbox-dev] Simple PCI device for shared memory

[Nev]

On Wed, 2010-05-19 at 16:54 +0200, Knut St. Osmundsen wrote:
> Nev wrote:
> [snip]
> > 4. call PDMDevHlpMMIO2Register - to allocate Host R3 address space ?
> 
> This allocates the actual physical memory as well as mapping it into the
> virtual address space of the VM process.  Note that you do NOT specify
> any address to this API, but you get a memory pointer back from it.  The
> guest will access the physical memory directly (via shadow page tables).
Thanks for info, But is there a more appropriate function? I have
searched and not been able to find one.
> 
> > 5. call PDMDevHlpMMHyperMapMMIO2 - to allocate Guest address space ?
> 
> This maps the memory into the hypervisor's raw-mode context.  This is
> only necessary if there is alternative ways of accessing the memory from
> the guest (the only use case is the VGA port I/O IIRC).
OK. Have deleted.
> 
> [snip]
> 
My requirement for shared memory is to be able to share a block of
memory between multiple Host processes and multiple guest processes.
To the best of my knowledge this in not possible on a Linux host unless
the memory is allocated using one of the IPC shared memory API such as
shm_open/mmap.
Searching thur the VBox code, memory is allocated with "malloc" or one
of the alloc variants, or mmap with MAP_ANONYMOUS.
Is this true?

I think I can patch the code to use mmap with MAP_SHARED. My
understanding of the code is very limited and would appreciate any
suggestion on the following.

I believe PDMDevHlpMMHyperMapMMIO2 calls
pfnMIO2Register which will call
pdmR3DevHelp_MMIO2Register which will call
PGMR3PhysMMIO2Register which will call
SUPR3PageAllocEx which will call
suplibOsIOCtl which issues an ioctl call, a which point I could not find
the target of this call.

But I think if I force the code to call supPageALlocNoKernelFallback,
instead of suplibOsIOCtl, when shared memory is required for Ring 3, I
can then modify the mmap call in suplibOsPageAlloc to map a shared
region.

Is this approach likely to be successful?