MAIR_EL2 Attributes

MAIR_EL2 Attributes

[Raz]

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1. According the documentation of MAIR_EL2 there are 8 attributes but
in the code you present only 6. Why ?
2. Why there are names to the attributes ? Does it make a difference if I
assign
an attribute index 2 or attribute index 0 in the page while the two attr

/*
 * Memory types available.
 */
#define MT_DEVICE_nGnRnE    0
#define MT_DEVICE_nGnRE        1
#define MT_DEVICE_GRE        2
#define MT_NORMAL_NC        3
#define MT_NORMAL        4
#define MT_NORMAL_WT        5

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Re: MAIR_EL2 Attributes

[Christoffer Dall]

Hi,

On Mon, Oct 23, 2017 at 09:33:28PM +0000, Raz wrote:
> 1. According the documentation of MAIR_EL2 there are 8 attributes but
> in the code you present only 6. Why ?

I just don't think we need those other defines.

> 2. Why there are names to the attributes ? Does it make a difference if I
> assign
> an attribute index 2 or attribute index 0 in the page while the two attr
> 

I'm not entirely sure what you're asking here.

Can you try to explain what the background of your question is?

I think these defines are just the way Linux configures the page tables.


> /*
>  * Memory types available.
>  */
> #define MT_DEVICE_nGnRnE    0
> #define MT_DEVICE_nGnRE        1
> #define MT_DEVICE_GRE        2
> #define MT_NORMAL_NC        3
> #define MT_NORMAL        4
> #define MT_NORMAL_WT        5

-Christoffer

Re: MAIR_EL2 Attributes

[Raz]

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On Tue, Oct 24, 2017 at 3:41 PM, Christoffer Dall <cdall@linaro.org> wrote:

> Hi,
>
> On Mon, Oct 23, 2017 at 09:33:28PM +0000, Raz wrote:
> > 1. According the documentation of MAIR_EL2 there are 8 attributes but
> > in the code you present only 6. Why ?
>
> I just don't think we need those other defines.
>
> > 2. Why there are names to the attributes ? Does it make a difference if I
> > assign
> > an attribute index 2 or attribute index 0 in the page while the two attr
> >
>
> I'm not entirely sure what you're asking here.
>
> Can you try to explain what the background of your question is?
>
Well, I wrote a thin hypervisor. The hypervisor is used to execute
protected code in a trusted environment , in this case EL2 is the trusted
environment.
The flow is as follows: Consider a function foo() :
..
int​ ​ foo()​ ​ {
   return​ ​ 19;
}
..

After g++ compilation:

_Z3foov():
​   ​ mov​ ​ ​ ​ ​ w0,​ ​ #0x13
​ ​ ​  ret

Now let say you want to run foo() in a trusted environment. To do that we
encrypt foo() and add its encrypted form as a new section to the program's
ELF. The old foo() is replaced
by opcodes that generates a trap.I chose brk:
_Z3foo():
brk
brk

Now, Whenever an encrypted program is executed , the moment it execute the
brk command, it traps into the hypervisor. The hypervisor decrypts foo and
executes it in EL2.
That's about it.

The essence of this idea is that I never let foo() code be visible to the
operating system.

My main obstacle now is performance ( I am 20 times slower) as at the
moment I have to flush the cache. I need to implement a cache write back
and this is done by manipulating mair_el2 and
the page descriptor.

Kind regards
Raz

>
>
> I think these defines are just the way Linux configures the page tables.
>
>
> > /*
> >  * Memory types available.
> >  */
> > #define MT_DEVICE_nGnRnE    0
> > #define MT_DEVICE_nGnRE        1
> > #define MT_DEVICE_GRE        2
> > #define MT_NORMAL_NC        3
> > #define MT_NORMAL        4
> > #define MT_NORMAL_WT        5
>
> -Christoffer
>



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