[Xen-users] Re: [Xen-devel] Xen - Guest memory allocation

[dinesh chandrasekaran <dinesh_chan8@xxxxxxxxxxx>]

Thank you Milos.

In     /xen/arch/x86/mm.c I modified the following snippet.

/* First 1MB of RAM is historically marked as I/O :    for ( i = 0; i < 0x100; i++ )
       share_xen_page_with_guest(mfn_to_page(i), dom_io, XENSHARE_writable);

    /* Any areas not specified as RAM by the e820 map are considered I/O. */    for ( i = 0, pfn = 0; pfn < max_page; i++ )
    {
         while ( (i < e820.nr_map) &&
                 (e820.map[i].type != E820_RAM) &&
                 (e820.map[i].type != E820_UNUSABLE) &&  (e820.map[i].type != E820_MY_PCI))

Then this region will not be marked I/O and I can thereby use populate_physmap() instead of iomem_access_permitted().
Regions other than E820_RAM, E820_UNUSABLE, E820_MY_PCI will thus belong to DOMID_IO and will require 
I/O permissions to be mapped by DomUs.

Is this perspective fine?

Thanks,
Dinesh C

> Date: Tue, 1 Jul 2008 10:46:12 +0100
> From: gm281@xxxxxxxxx
> To: dinesh_chan8@xxxxxxxxxxx
> Subject: Re: [Xen-users] Re: [Xen-devel] Xen - Guest memory allocation
>
> > Okay.
> > But I want to use 'I/O pages: memory in the memory mapped IO
> > regions obtained by reading the BAR value from the pci device'
> > as the 'real' memory for all DomUs since I have 256MB behind that device.
>
> The problem you'll have to face is the ownership model that Xen uses.
> By default when RAM is allocated to a domain it is only accessible to
> the domain itself + Dom0 (via foreign mapping mechanism). All non-RAM
> ranges are allocated to a synthetic domain called dom_io. Any
> pagetables that reference this memory are verified against the
> permitted iomem range (look iomem_access_permitted()). By default
> Dom0's access is permitted, so you are fine there. However, you'll
> have to teach the domain build tools to recognise the difference
> between RAM and MMIO (it might be as simple as patching
> direct_remap_pfn_range in linux kernel to use DOMID_IO). Also, another
> concern I'd have, is if the CPU will be happy to use MMIO for code and
> stack segments. I don't know enough about that to answer.
>
> CHeers
> Gr(z)egor(z)

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