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Re: [Xen-devel] Implementing split memory in Xen is annoyingly hard?

  • To: Sina Bahram <sbahram@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
  • Date: Fri, 12 Dec 2008 09:54:36 +0000
  • Cc:
  • Delivery-date: Fri, 12 Dec 2008 01:55:04 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AclcJXuNUVD+QGa8QgqZc2JyDmh1sQAFVHONAACwI2AAAIWDoQ==
  • Thread-topic: [Xen-devel] Implementing split memory in Xen is annoyingly hard?

Well, you can trap all everything if you make the page not present.

Beyond that you can selectively make a page writable or executable, but in
these cases it will have to be readable too.

 -- Keir

On 12/12/2008 09:50, "Sina Bahram" <sbahram@xxxxxxxxx> wrote:

> I'm not trying to make the page executable  but not readable, as long as I
> can trap all reads, that's all.
> I'll be implementing a split TLB which will handle the fact that all
> executes get shunted to a "split page".
> Another way of saying that is that if someone looks up a virtual address, it
> gets translated to one physical address if it's for read/writes and to
> another one for executes. Sure one can execute the page that is read/write
> and someone could read/write to the page that is execute, but it will never
> happen because I'd never translate it that way in my code.
> Btw, this completely eliminates anything like lisp, javascript, or anything
> else from running, as they run code they have written all the time, but
> that's why I only plan on doing it for kernel pages which don't write things
> that they then execute.
> Hope that's more clear, and is this possible, do you think, or is my summary
> in the original email accurate in that it can't be done easily in xen?
> Take care,
> Sina
> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Friday, December 12, 2008 4:20 AM
> To: Sina Bahram; xen-devel@xxxxxxxxxxxxxxxxxxx;
> xen-research@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Implementing split memory in Xen is annoyingly
> hard?
> On 12/12/2008 06:47, "Sina Bahram" <sbahram@xxxxxxxxx> wrote:
>> #2: Xen absolutely does not, (can not?), cause a fault or other VM exit to
>> be generated upon an execute or a read of a page.
> It's not possible to make a page executable but not readable, so indeed I
> think you're stumped there, unless you can work out a hackish way to
> desynchronise the iTLB and the dTLB (an operation not supported
> architecturally by x86 of course).
>  -- Keir

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