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

[Khalid Aziz <khalid.aziz@oracle.com>]

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

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