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

[Rob Gardner]

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@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.
>
> 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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