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

[Khalid Aziz]

On 03/08/2016 12:57 PM, David Miller wrote:
> From: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> Date: Mon, 7 Mar 2016 14:06:43 -0700
>
> > Good questions. Isn't set of valid VAs already constrained by VA_BITS
> > (set to 44 in arch/sparc/include/asm/processor_64.h)? As I see it we
> > are already not using the top 4 bits. Please correct me if I am wrong.
>
> Another limiting constraint is the number of address bits coverable by
> the 4-level page tables we use.  And this is sign extended so we have
> a top-half and a bottom-half with a "hole" in the center of the VA
> space.
>
> I want some clarification on the top bits during ADI accesses.
>
> If ADI is enabled, then the top bits of the virtual address are
> intepreted as tag bits.  Once "verified" with the ADI settings, what
> happense to these tag bits?  Are they dropped from the virtual address
> before being passed down the TLB et al. for translations?

Bits 63-60 (tag bits) are dropped from the virtual address before being 
passed down the TLB for translation when PSTATE.mcde = 1.

--
Khalid

> If not, then this means you have to map ADI memory to the correct
> location so that the tags match up.
>
> And if that's the case, if you really wanted to mix tags within a
> single page, you'd have to map that page several times, once for each
> and every cacheline granular tag you'd like to use within that page.