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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