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

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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@xxxxxxxxxxxxxxx>
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.

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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