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xen-users
RE: [Xen-users] VIRT to MACHINE conversion
> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Ulrich Windl
> Sent: 10 January 2007 10:22
> To: Security Initiative Team
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] VIRT to MACHINE conversion
>
> On 9 Jan 2007 at 17:47, Security Initiative Team wrote:
>
> > I'm confused about the relation between
> > virt and phy addresses in a guest domain?
> > What is the co-relation between the two?
> >
The guest will manage it's own virtual memory mappings. However, those
are guest physical addresses, so the guest will think that it's got
0..256M address range, whilst the machine physical address is, say,
256M..512M.
So there is, essentially, a second level of indirection. This is managed
by the shadow page table code in Xen - this works in essence by having
two different page-tables, one that the guest updates, one that is
maintained by the SPT code. The SPT table will be the one that the
processor actually uses.
However, xenified guests have the ability to actually know that machine
and guest physical memory isn't the same, so some of that will be done
with direct hypercalls, bypassing the shadow page table code.
>
> I don't know as well, but I guess as the guest domain has no
> physical RAM, the
> mapping is 1:1 (the "physical RAM" in DomU is actually virtual)
That's not correct. The DomU doesn't necessarily have the same machine
physical address as it "thinks", but it doesn't mean that the mapping is
1:1 - it just means that there is a second layer of mapping, which for
most purposes is a "trivial" mapping, guest physical page to machine
physical page.
--
Mats
>
> Ulrich
>
>
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> Xen-users mailing list
> Xen-users@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-users
>
>
>
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