dinesh chandrasekaran
2008-Jul-08 19:24 UTC
[Xen-users] Re: [Xen-devel] Xen - Guest memory allocation
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@cam.ac.uk
> To: dinesh_chan8@hotmail.com
> 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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