Re: [PATCH v2] sparc64: Add support for Application Data Integrity (ADI)

[Rob Gardner]

On 03/07/2016 01:38 PM, David Miller wrote:
> From: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> Date: Mon, 7 Mar 2016 14:33:56 -0700
>
> > On 03/07/2016 12:16 PM, David Miller wrote:
> > > From: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> > > Date: Mon, 7 Mar 2016 11:24:54 -0700
> > >
> > > > Tags can be cleared by user by setting tag to 0. Tags are
> > > > automatically cleared by the hardware when the mapping for a virtual
> > > > address is removed from TSB (which is why swappable pages are a
> > > > problem), so kernel does not have to do it as part of clean up.
> > > You might be able to crib some bits for the Tag in the swp_entry_t,
> > > it's
> > > 64-bit and you can therefore steal bits from the offset field.
> > >
> > > That way you'll have the ADI tag in the page tables, ready to
> > > re-install
> > > at swapin time.
> > That is a possibility but limited in scope. An address range covered
> > by a single TTE can have large number of tags. Version tags are set on
> > cacheline. In extreme case, one could set a tag for each set of
> > 64-bytes in a page. Also tags are set completely in userspace and no
> > transition occurs to kernel space, so kernel has no idea of what tags
> > have been set. I have not found a way to query the MMU on tags.
> >
> > I will think some more about it.
> That would mean that ADI is impossible to use for swappable memory.
>
> ...
>
> If that's true I'm extremely disappointed that they devoted so much
> silicon and engineering to this feature yet didn't take that one
> critical step to make it generally useful. :(

You can easily read ADI tags with a simple ldxa #ASI_MCD_PRIMARY 
instruction.

Rob