RE: [Xen-users] VIRT to MACHINE conversion

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: Security Initiative Team [mailto:passrete@xxxxxxxxx] 
> Sent: 10 January 2007 13:28
> To: Petersson, Mats; Ulrich Windl
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-users] VIRT to MACHINE conversion
> 
> In the default mode of operation, doesn't Xen work without SPTs?

Xenified guests (non-HVM) do indeed write directly to the page-table via
the Hypercalls. 

> 
> The addresses entered into the PTs of the guest are actual 
> machine addresses,
> right?
> So the PTs remain a virtual-to-machine address mapping,
> so where do the (pseudo) physical addresses come into play.

That's the HVM way of working - since OS's need memory to start "at
zero", the hypervisor will have a three-level translation.
> 
> Also, when a guest domain wants to update the PT of a process,
> (say to change the permissions from read-only to writable),
> what is the exact parameter that it sends to the hypervisor --
> page frame number ?

Look at the code in xen/arch/x86/mm.c:do_mmu_update and you'll be at a
good starting point. 

--
Mats
> 
> 
> 
> "Petersson, Mats" <Mats.Petersson@xxxxxxx> 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. 
>       
>       
> 
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