Mini-OS and Xen!

Mini-OS and Xen!

[Jayaraman, Bhaskar]

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Hi I have a doubt regarding Mini-OS and Xen. If it is true that Xen reserves in the virtual address space the top 64MB on 32bit systems and 168MB in PAE systems, doesn't it exist with a Mini-OS kernel which has only 32MB in size i.e. going by its default config file? Obviously I'm missing something.
Regards,
Bhaskar.

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Re: Mini-OS and Xen!

[Daniel Stodden]

On Tue, 2008-08-05 at 14:01 +0800, Jayaraman, Bhaskar wrote:
> Hi I have a doubt regarding Mini-OS and Xen. If it is true that Xen
> reserves in the virtual address space the top 64MB on 32bit systems
> and 168MB in PAE systems, doesn’t it exist with a Mini-OS kernel which
> has only 32MB in size i.e. going by its default config file? Obviously
> I’m missing something.

Virtual address space size is always a full 32bit or 64bit,
respectively. That's not yet memory, but mere address space.

Parts of which are mapped to a hypervisor and a guest system,
respectively. Now it's memory.

Therein, guest system size does not include hypervisor size.

Best,
Daniel

-- 
Daniel Stodden
LRR     -      Lehrstuhl für Rechnertechnik und Rechnerorganisation
Institut für Informatik der TU München             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@cs.tum.edu
PGP Fingerprint: F5A4 1575 4C56 E26A 0B33  3D80 457E 82AE B0D8 735B

RE: Mini-OS and Xen!

[Jayaraman, Bhaskar]

Ok so what you're saying is that if the hypervisor size ever bloats up to 64MB then it'll show up in the size used up by the VM. But since the hypervisor in our case is still under .5MB in size it will fit in well within the 32 MB space reserved for Mini-OS? Although it is addressed by virtual addresses which are beyond the 32 MB range.

Now if the virtual address space of the VM is used up to reserve the GDT pages then I would assume that the actual available number of page frames will be less than 32MB/4KB for the VM? To be precise 32MB/4KB - .5MB(approx Xen)/4KB?
Also, since GDT pages 15 and 16 are used up by the hypervisor, does it mean Xen will have one Xen kernel segment for each of the VMs that is spawned? What will be those segments per VM e.g. kernel stack, data, code?
If so what can be the theoretical maximum number of VMs that we can spawn with the 2 GDT pages available to Xen? i.e. 1024 entries that are 8 bytes long.

Regards,
Bhaskar.

-----Original Message-----
From: Daniel Stodden [mailto:stodden@cs.tum.edu]
Sent: Tuesday, August 05, 2008 11:54 AM
To: Jayaraman, Bhaskar
Cc: Xen Developers
Subject: Re: [Xen-devel] Mini-OS and Xen!

On Tue, 2008-08-05 at 14:01 +0800, Jayaraman, Bhaskar wrote:
> Hi I have a doubt regarding Mini-OS and Xen. If it is true that Xen
> reserves in the virtual address space the top 64MB on 32bit systems
> and 168MB in PAE systems, doesn't it exist with a Mini-OS kernel which
> has only 32MB in size i.e. going by its default config file? Obviously
> I'm missing something.

Virtual address space size is always a full 32bit or 64bit,
respectively. That's not yet memory, but mere address space.

Parts of which are mapped to a hypervisor and a guest system,
respectively. Now it's memory.

Therein, guest system size does not include hypervisor size.

Best,
Daniel

--
Daniel Stodden
LRR     -      Lehrstuhl für Rechnertechnik und Rechnerorganisation
Institut für Informatik der TU München             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@cs.tum.edu
PGP Fingerprint: F5A4 1575 4C56 E26A 0B33  3D80 457E 82AE B0D8 735B

Re: Mini-OS and Xen!

[Samuel Thibault]

Jayaraman, Bhaskar, le Tue 05 Aug 2008 15:24:14 +0800, a écrit :
> If so what can be the theoretical maximum number of VMs that we can spawn with the 2 GDT pages available to Xen? i.e. 1024 entries that are 8 bytes long.

Xen switches GDT.

Samuel

RE: Mini-OS and Xen!

[Jayaraman, Bhaskar]

Samuel, this means the Ring 0 pages are shared among all the VMs that are spawned. Meaning the kernel segment descriptors of each GDT of every VM will contain virtual addresses which all point to the same physical address, of that of Xen eliminating replication.
Bhaskar.

-----Original Message-----
From: Samuel Thibault [mailto:samuel.thibault@eu.citrix.com]
Sent: Tuesday, August 05, 2008 3:24 PM
To: Jayaraman, Bhaskar
Cc: Daniel Stodden; Xen Developers
Subject: Re: [Xen-devel] Mini-OS and Xen!

Jayaraman, Bhaskar, le Tue 05 Aug 2008 15:24:14 +0800, a écrit :
> If so what can be the theoretical maximum number of VMs that we can spawn with the 2 GDT pages available to Xen? i.e. 1024 entries that are 8 bytes long.

Xen switches GDT.

Samuel