Messages in this thread | | | Subject | Re: [RFC PATCH v3] sparc64: Add support for Application Data Integrity (ADI) | From | Rob Gardner <> | Date | Fri, 6 Jan 2017 10:28:44 -0800 |
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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 >> >> -- >> To unsubscribe from this list: send the line "unsubscribe sparclinux" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- > To unsubscribe from this list: send the line "unsubscribe sparclinux" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
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