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Re: [Xen-devel] Xen memory management

To: David Xu <davidxu06@xxxxxxxxx>
Subject: Re: [Xen-devel] Xen memory management
From: Srujan Kotikela <ksrujandas@xxxxxxxxx>
Date: Thu, 23 Jun 2011 23:52:15 -0500
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Jonathan Tripathy <jonnyt@xxxxxxxxxxx>
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On Thu, Jun 23, 2011 at 8:37 PM, David Xu <davidxu06@xxxxxxxxx> wrote:
 On a shared-memory system with multi-core cpu, can one VM occupy all cache and prevent other VMs using cache efficiently?

Thanks for reply from all of you. I am reading a paper which tells some secure problem of Xen VMM. I am not familiar with something that is related to those problems. So I really need your help. Of course, please feel free to post your opinion. Anybody is welcome to have a discuss.

2011/6/23 Jonathan Tripathy <jonnyt@xxxxxxxxxxx>

On 24/06/2011 00:50, David Xu wrote:

2011/6/23 Jonathan Tripathy <jonnyt@xxxxxxxxxxx>

On 23/06/2011 23:08, David Xu wrote:
Thanks. My concern is that if several VMs are mapped to same memory, one VM may get something from the memory which has ever been used by another VM. This may cause some secure problems. 

Someone correct me if I'm wrong, but I'm pretty sure that a DomU kernel (If the flag is set correctly during compile time) will scrub (i.e. "zero") RAM first before releasing it to the Xen Hypervisor. Then hypervisor will then subsequently assign that bit of RAM to another domain.

Sounds good. Does Xen VMM can control the mapping between a part of memory and cache line? That is to say I wander whether Xen can guarantee different VMs will use different cache line. Thanks.


Please don't top post :)

I'm not a Xen dev, so it would be great if a dev could let me know if I'm talking rubbish or not. However from my very limited knowledge of how CPU caches work (which comes from basic single CPU, non VMM related system), common sense would tell me that the cache line would be different for each DomU, as a CPUs cache is inherently linked to main memory (RAM). I believe that the process used to access data from memory is abstracted by the CPU, so assuming that Xen prevents access to RAM from another DomU, I guess it would make sense to say that any data that is cached in the CPU is protected.

Then again, I could be completly wrong......

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