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

[Andy Lutomirski]

On Mon, Mar 7, 2016 at 7:30 AM, Rob Gardner <rob.gardner@xxxxxxxxxx> wrote:
> On 03/07/2016 07:07 AM, Khalid Aziz wrote:
>>
>> On 03/05/2016 09:07 PM, David Miller wrote:
>>>
>>> From: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
>>> Date: Wed,  2 Mar 2016 13:39:37 -0700
>>>
>>>>     In this
>>>>     first implementation I am enabling ADI for hugepages only
>>>>     since these pages are locked in memory and hence avoid the
>>>>     issue of saving and restoring tags.
>>>
>>>
>>> This makes the feature almost entire useless.
>>>
>>> Non-hugepages must be in the initial implementation.
>>
>>
>> Hi David,
>>
>> Thanks for the feedback. I will get this working for non-hugepages as
>> well. ADI state of each VMA region is already stored in the VMA itself in my
>> first implementation, so I do not lose it when the page is swapped out. The
>> trouble is ADI version tags for each VMA region have to be stored on the
>> swapped out pages since the ADI version tags are flushed when TLB entry for
>> a page is flushed.
>
>
>
> Khalid,
>
> Are you sure about that last statement? My understanding is that the tags
> are stored in physical memory, and remain there until explicitly changed or
> removed, and so flushing a TLB entry has no effect on the ADI tags. If it
> worked the way you think, then somebody would have to potentially reload a
> long list of ADI tags on every TLB miss.
>

I'll bite, since this was sent to linux-api:

Can someone explain what this feature does for the benefit of people
who haven't read the manual (and who don't even know where to find the
manual)?

Are the top few bits of a sparc64 virtual address currently
must-be-zero?  Does this feature change the semantics so that those
bits are ignored for address resolution and instead must match
whatever the ADI tag is determined to be during address resolution?

Is this enforced for both user and kernel accesses?

Is the actual ADI tag associated with a "page" associated with the
page of physical memory or is it associated with a mapping?  That is,
if there are two virtual aliases of the same physical page (in the
same process or otherwise), does the hardware require them to have the
same ADI tag?  If the answer is no, then IMO this is definitely
something that should use mprotect and you should seriously consider
using something like mprotect_key (new syscall, not in Linus' tree
yet) for it.  In fact, you might consider a possible extra parameter
to that syscall for this purpose.

Cc: Dave Hansen.  It seems to be the zeitgeist to throw tag bits at
PTEs these days.