Re: pkeys: Reserve PKEY_DISABLE_READ

[Ram Pai <linuxram@us.ibm.com>]

On Thu, Nov 08, 2018 at 09:23:35PM +0100, Florian Weimer wrote:
> * Ram Pai:
> 
> > Florian,
> >
> > 	I can. But I am struggling to understand the requirement. Why is
> > 	this needed?  Are we proposing a enhancement to the sys_pkey_alloc(),
> > 	to be able to allocate keys that are initialied to disable-read
> > 	only?
> 
> Yes, I think that would be a natural consequence.
> 
> However, my immediate need comes from the fact that the AMR register can
> contain a flag combination that is not possible to represent with the
> existing PKEY_DISABLE_WRITE and PKEY_DISABLE_ACCESS flags.  User code
> could write to AMR directly, so I cannot rule out that certain flag
> combinations exist there.
> 
> So I came up with this:
> 
> int
> pkey_get (int key)
> {
>   if (key < 0 || key > PKEY_MAX)
>     {
>       __set_errno (EINVAL);
>       return -1;
>     }
>   unsigned int index = pkey_index (key);
>   unsigned long int amr = pkey_read ();
>   unsigned int bits = (amr >> index) & 3;
> 
>   /* Translate from AMR values.  PKEY_AMR_READ standing alone is not
>      currently representable.  */
>   if (bits & PKEY_AMR_READ)

this should be
   if (bits & (PKEY_AMR_READ|PKEY_AMR_WRITE))


>     return PKEY_DISABLE_ACCESS;


>   else if (bits == PKEY_AMR_WRITE)
>     return PKEY_DISABLE_WRITE;
>   return 0;
> }
> 
> And this is not ideal.  I would prefer something like this instead:
> 
>   switch (bits)
>     {
>       case PKEY_AMR_READ | PKEY_AMR_WRITE:
>         return PKEY_DISABLE_ACCESS;
>       case PKEY_AMR_READ:
>         return PKEY_DISABLE_READ;
>       case PKEY_AMR_WRITE:
>         return PKEY_DISABLE_WRITE;
>       case 0:
>         return 0;
>     }

yes.
 and on x86 it will be something like:
   switch (bits)
     {
       case PKEY_PKRU_ACCESS :
         return PKEY_DISABLE_ACCESS;
       case PKEY_AMR_WRITE:
         return PKEY_DISABLE_WRITE;
       case 0:
         return 0;
     }

But for this to work, why do you need to enhance the sys_pkey_alloc()
interface?  Not that I am against it. Trying to understand if the
enhancement is really needed.

> 
> By the way, is the AMR register 64-bit or 32-bit on 32-bit POWER?

It is 64-bit.

RP