Re: [RFC PATCH v3] sparc64: Add support for Application Data Integrity (ADI)

[Khalid Aziz]

On 01/04/2017 05:14 PM, Dave Hansen wrote:
> On 01/04/2017 04:05 PM, Rob Gardner wrote:
> > > What if two different small pages have different tags and khugepaged
> > > comes along and tries to collapse them?  Will the page be split if a
> > > user attempts to set two different tags inside two different small-page
> > > portions of a single THP?
> >
> > The MCD tags operate at a resolution of cache lines (64 bytes). Page
> > sizes don't matter except that each virtual page must have a bit set in
> > its TTE to allow MCD to be enabled on the page. Any page can have many
> > different tags, one for each cache line.
>
> Is an "MCD tag" the same thing as a "ADI version tag"?
>
> The thing that confused me here is that we're taking an entire page of
> "ADI version tags" and stuffing them into a swap pte (in
> set_swp_pte_at()).  Do we somehow have enough space in a swap pte on
> sparc to fit PAGE_SIZE/64 "ADI version tag"s in there?

No, we do not have space to stuff PAGE_SIZE/64 version tags in swap pte. 
There is enough space for just one tag per page. DaveM had suggested 
doing this since the usual case is for a task to set one tag per page 
even though MMU does not require it. I have implemented this as first 
pass to start a discussion and get feedback on whether rest of the 
swapping implementation and other changes look right, hence the patch is 
"RFC". If this all looks good, I can expand swapping support in a 
subsequent patch or iteration of this patch to allocate space in 
mm_context_t possibly to store per cacheline tags. I am open to any 
other ideas on storing this larger number of version tags.

Thanks,
Khalid