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

[Rob Gardner <rob.gardner@oracle.com>]

On 01/06/2017 10:18 AM, Khalid Aziz wrote:
> On 01/06/2017 10:54 AM, Rob Gardner wrote:
>> On 01/06/2017 09:10 AM, Khalid Aziz wrote:
>>> On 01/06/2017 10:02 AM, David Miller wrote:
>>>> From: Dave Hansen <dave.hansen@linux.intel.com>
>>>> Date: Fri, 6 Jan 2017 08:55:03 -0800
>>>>
>>>>> Actually, that reminds me...  How does your code interface with
>>>>> ksm?  Or
>>>>> is there no interaction needed since you're always working on virtual
>>>>> addresses?
>>>>
>>>> This reminds me, I consider this feature potentially extremely useful
>>>> for
>>>> kernel debugging.  So I would like to make sure we don't implement
>>>> anything
>>>> in a way which would preclude that in the long term.
>>>
>>> I agree and please do point out if I have made any implementation
>>> decisions that could preclude that.
>>>
>>> Thanks,
>>> Khalid
>>
>>
>> Khalid, I have already pointed out an implementation decision that
>> interferes with the potential for kernel debugging with ADI: lazy
>> clearing of version tags.
>
> This does not preclude kernel debugging. If kernel debugging ends up 
> requiring tags be cleared whenever a page is freed, we can add that 
> code as part of kernel debugging support code and enable it 
> conditionally only when kernel is being debugged. Forcing every task 
> to incur the large cost of clearing tags on every "free" all the time 
> is just not an acceptable cost only to support kernel debugging. It 
> should be a dynamic switch to be toggled on only when debugging 
> kernel. PSTATE.mcde being set is not enough to trigger a trap. It is 
> easy enough to clear TTE.mcd before block initialization of a page and 
> avoid a trap due to tag mismatch, or just use physical address with 
> block initialization.
>
> We can evaluate all of these options when we get to implementing 
> kernel debugging using ADI.

I didn't say it precludes kernel debugging, just that it interferes, and 
there will be additional work to do if we want kernel debugging 
capability with ADI.

Rob



>
> Thanks,
> Khalid
>
>
>>
>> Details: when memory is "freed" the version tags are left alone, as it
>> is an expensive operation to go through the memory and clear the tag for
>> each cache line. So this is done lazily whenever memory is "allocated".
>> More specifically, the first time a user process touches freshly
>> allocated memory, a fault occurs and the kernel then clears the page. In
>> the NG4 and M7 variants of clear_user_page, the block init store ASI is
>> used to optimize, and it has the side effect of clearing the ADI tag for
>> the cache line. BUT only if pstate.mcde is clear. If pstate.mcde is set,
>> then instead of the ADI tag being cleared, the tag is *checked*, and if
>> there is a mismatch between the version in the virtual address and the
>> version in memory, then you'll get a trap and panic. Therefore, with
>> this design, you cannot have pstate.mcde enabled while in the kernel (in
>> general). To solve this you have to check the state of pstate.mcde (or
>> just turn it off) before doing any block init store in clear_user_page,
>> memset, memcpy, etc.
>>
>> Rob
>>
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