doubt on domU segment mappings

doubt on domU segment mappings

[Sandesh]

Hi all,
  Can someone please tell me if the segment mappings like kernel_seg,
ramdisk_seg, pgtables_seg, start_info_seg, etc.. are static or are they
subjected to change, in the sense the are the page table entries in dom0
mapping these segments change or do they remain static?? 
  Any hints or reference regarding this is greatly appreciated.

Thanks in advance.

Regards,
  Sandesh

  


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Re: doubt on domU segment mappings

[Samuel Thibault]

Sandesh, le Mon 21 Jul 2008 22:20:28 +0530, a écrit :
>   Can someone please tell me if the segment mappings like kernel_seg,
> ramdisk_seg, pgtables_seg, start_info_seg, etc.. are static or are they
> subjected to change, in the sense the are the page table entries in dom0
> mapping these segments change or do they remain static?? 

These are constructed by the PV domain builder just for the domU.  DomU
is then free to do whatever it wants with it, dom0 doesn't care at all.
In the PV-GRUB case for instance, it is completely rebuilt from within
the domU.

Samuel

Re: doubt on domU segment mappings

[Sandesh]

thanks for the reply...

> These are constructed by the PV domain builder just for the domU.
> DomU
> is then free to do whatever it wants with it, dom0 doesn't care at
> all 
but aren't these mappings in the dom0 page tables?? Once the domU is
running how can i change them? like how will i know which entries in the
dom0 pgtables corresponds to kernel_seg or ramdisk_seg and so on..

Regards,
 Sandesh


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Re: doubt on domU segment mappings

[Samuel Thibault]

Sandesh, le Mon 21 Jul 2008 22:34:33 +0530, a écrit :
> > These are constructed by the PV domain builder just for the domU.
> > DomU
> > is then free to do whatever it wants with it, dom0 doesn't care at
> > all 
> but aren't these mappings in the dom0 page tables?

Just temporarily when the domain builder fills them, but as soon as it's
built, they are unmapped.

> Once the domU is running how can i change them?

What for?

> like how will i know which entries in the dom0 pgtables corresponds to
> kernel_seg or ramdisk_seg and so on..

Again, dom0 doesn't map them any more, it doesn't need them any more.

Samuel

Re: doubt on domU segment mappings

[Sandesh]

On Mon, 2008-07-21 at 17:55 +0100, Samuel Thibault wrote:
> Sandesh, le Mon 21 Jul 2008 22:34:33 +0530, a écrit :
> > > These are constructed by the PV domain builder just for the domU.
> > > DomU
> > > is then free to do whatever it wants with it, dom0 doesn't care at
> > > all
> > but aren't these mappings in the dom0 page tables?
>
> Just temporarily when the domain builder fills them, but as soon as it's
> built, they are unmapped.
>
> > Once the domU is running how can i change them?
>
> What for?
basically right after shutdown with poweroff = 'preserve' option i want
to reboot the domU without having to going through the whole domain
building process. So i need to load the different domU segments again,
or at least the pt_base mapping.
>
> > like how will i know which entries in the dom0 pgtables corresponds to
> > kernel_seg or ramdisk_seg and so on..
>
> Again, dom0 doesn't map them any more, it doesn't need them any more.
then how will the domU kernel memory is accessed? are they mapped to the
domU pg_tables?
>
> Samuel


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Re: doubt on domU segment mappings

[Samuel Thibault]

Sandesh, le Mon 21 Jul 2008 22:43:13 +0530, a écrit :
> > > Once the domU is running how can i change them?
> >
> > What for?
> basically right after shutdown with poweroff = 'preserve' option i want
> to reboot the domU without having to going through the whole domain
> building process. So i need to load the different domU segments again,
> or at least the pt_base mapping.

Well the problem is that the domU most probably has destroyed it
completely already.

> > > like how will i know which entries in the dom0 pgtables corresponds to
> > > kernel_seg or ramdisk_seg and so on..
> >
> > Again, dom0 doesn't map them any more, it doesn't need them any more.
> then how will the domU kernel memory is accessed? are they mapped to the
> domU pg_tables?

Yes.  No need for anybody else to access it (except PV interfaces, but
then the MFNs or grants are explicitely given by the domU).

Samuel

Re: doubt on domU segment mappings

[Sandesh]

On Mon, 2008-07-21 at 18:08 +0100, Samuel Thibault wrote:
> > or at least the pt_base mapping.
> 
> Well the problem is that the domU most probably has destroyed it
> completely already.
can't i preserve them, at least the base pointers??? :)

> > > Again, dom0 doesn't map them any more, it doesn't need them any
> more.
> > then how will the domU kernel memory is accessed? are they mapped to
> the
> > domU pg_tables?
so even pt_base becomes invalid once domU has booted?? 
can u just point to me where these segment mappings are recreated in
domU(linux 2.6.18.1).

Regards,
 Sandesh


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Re: doubt on domU segment mappings

[Samuel Thibault]

Sandesh, le Mon 21 Jul 2008 23:06:31 +0530, a écrit :
> On Mon, 2008-07-21 at 18:08 +0100, Samuel Thibault wrote:
> > > or at least the pt_base mapping.
> > 
> > Well the problem is that the domU most probably has destroyed it
> > completely already.
> can't i preserve them, at least the base pointers??? :)
> can u just point to me where these segment mappings are recreated in
> domU(linux 2.6.18.1).

Linux domU starts in head-xen.S's startup_32, which calls start_kernel,
which itself becomes the idle thread, so I guess it actually preserves
it.  As for the pt_base address, you can get it from linux itself from
the xen_start_info structure.  There is no way to get that from other
domains.

> > > > Again, dom0 doesn't map them any more, it doesn't need them any
> > more.
> > > then how will the domU kernel memory is accessed? are they mapped to
> > the
> > > domU pg_tables?
> so even pt_base becomes invalid once domU has booted?? 

DomU is free to do so, yes.

Samuel

Re: doubt on domU segment mappings

[Sandesh]

On Mon, 2008-07-21 at 17:55 +0100, Samuel Thibault wrote:
> Sandesh, le Mon 21 Jul 2008 22:34:33 +0530, a écrit :
> > > These are constructed by the PV domain builder just for the domU.
> > > DomU
> > > is then free to do whatever it wants with it, dom0 doesn't care at
> > > all
> > but aren't these mappings in the dom0 page tables?
>
> Just temporarily when the domain builder fills them, but as soon as it's
> built, they are unmapped.
hi samuel,
 can u just point me where the segments are being unmapped??
so far i have not come across the code achieving this functionality.

Thanks a lot,

Regards,
 Sandesh




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Re: doubt on domU segment mappings

[Samuel Thibault]

Sandesh, le Tue 22 Jul 2008 00:21:54 +0530, a écrit :
> On Mon, 2008-07-21 at 17:55 +0100, Samuel Thibault wrote:
> > Sandesh, le Mon 21 Jul 2008 22:34:33 +0530, a écrit :
> > > > These are constructed by the PV domain builder just for the domU.
> > > > DomU
> > > > is then free to do whatever it wants with it, dom0 doesn't care at
> > > > all
> > > but aren't these mappings in the dom0 page tables?
> >
> > Just temporarily when the domain builder fills them, but as soon as it's
> > built, they are unmapped.
>  can u just point me where the segments are being unmapped??

xc_dom_release calls xc_dom_unmap_all()

> so far i have not come across the code achieving this functionality.

you should probably tune your grep.

Samuel

Re: doubt on domU segment mappings

[Sandesh]

On Mon, 2008-07-21 at 22:34 +0530, Sandesh wrote:
> thanks for the reply...
> 
> > These are constructed by the PV domain builder just for the domU.
> > DomU
> > is then free to do whatever it wants with it, dom0 doesn't care at
> > all 
> but aren't these mappings in the dom0 page tables?? Once the domU is
> running how can i change them? like how will i know which entries in the
> dom0 pgtables corresponds to kernel_seg or ramdisk_seg and so on..
i want to access these mappings from the hypervisor once domU is booted,
not from domU guest kernel. So in that perspective how will i know which
entries point to respective segments. 

> 
> Regards,
>  Sandesh


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Re: doubt on domU segment mappings

[Samuel Thibault]

Sandesh, le Mon 21 Jul 2008 22:38:15 +0530, a écrit :
> On Mon, 2008-07-21 at 22:34 +0530, Sandesh wrote:
> > thanks for the reply...
> > 
> > > These are constructed by the PV domain builder just for the domU.
> > > DomU
> > > is then free to do whatever it wants with it, dom0 doesn't care at
> > > all 
> > but aren't these mappings in the dom0 page tables?? Once the domU is
> > running how can i change them? like how will i know which entries in the
> > dom0 pgtables corresponds to kernel_seg or ramdisk_seg and so on..
> i want to access these mappings from the hypervisor once domU is booted,

What for?  What is your domU guest?

Samuel

Re: doubt on domU segment mappings

[Sandesh]

On Mon, 2008-07-21 at 18:00 +0100, Samuel Thibault wrote:
> Sandesh, le Mon 21 Jul 2008 22:38:15 +0530, a écrit :
> > On Mon, 2008-07-21 at 22:34 +0530, Sandesh wrote:
> > > thanks for the reply...
> > >
> > > > These are constructed by the PV domain builder just for the domU.
> > > > DomU
> > > > is then free to do whatever it wants with it, dom0 doesn't care at
> > > > all
> > > but aren't these mappings in the dom0 page tables?? Once the domU is
> > > running how can i change them? like how will i know which entries in the
> > > dom0 pgtables corresponds to kernel_seg or ramdisk_seg and so on..
> > i want to access these mappings from the hypervisor once domU is booted,
>
> What for?  What is your domU guest?
domU guest is linux 2.8.18.1 hypervisor kernel.
>
> Samuel


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