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From: Florian Weimer <fweimer@redhat.com>
Subject: Re: pkeys: Reserve PKEY_DISABLE_READ
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Date: Thu, 08 Nov 2018 21:23:35 +0100
In-Reply-To: <20181108201231.GE5481@ram.oc3035372033.ibm.com> (Ram Pai's
	message of "Thu, 8 Nov 2018 12:12:31 -0800")
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To: Ram Pai <linuxram@us.ibm.com>
Cc: Dave Hansen <dave.hansen@intel.com>, linux-api@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org

* 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)
    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;
    }

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

Thanks,
Florian