RE: Implementing split memory in Xen is annoyingly hard?

["Sina Bahram" <sbahram@nc.rr.com>]

But there's no straight forward way of making the page not exist each time,
is there?

Take care,
Sina

-----Original Message-----
From: Keir Fraser [mailto:keir.fraser@eu.citrix.com] 
Sent: Friday, December 12, 2008 4:55 AM
To: Sina Bahram; xen-devel@lists.xensource.com
Subject: Re: [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@nc.rr.com> 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@eu.citrix.com]
> Sent: Friday, December 12, 2008 4:20 AM
> To: Sina Bahram; xen-devel@lists.xensource.com;
> xen-research@lists.xensource.com
> Subject: Re: [Xen-devel] Implementing split memory in Xen is annoyingly
> hard?
> 
> On 12/12/2008 06:47, "Sina Bahram" <sbahram@nc.rr.com> 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
> 
>