Hi Robin,
On 2020-06-08 16:56, Robin Murphy wrote:
On 2020-06-08 10:13, Sai Prakash Ranjan wrote:
Hi Will,
On 2020-06-08 13:48, Will Deacon wrote:
On Sun, Jun 07, 2020 at 04:39:18PM +0530, Sai Prakash Ranjan wrote:
Remove SMMU shutdown callback since it seems to cause more
problems than benefits. With this callback, we need to make
sure that all clients/consumers of SMMU do not perform any
DMA activity once the SMMU is shutdown and translation is
disabled. In other words we need to add shutdown callbacks
for all those clients to make sure they do not perform any
DMA or else we see all kinds of weird crashes during reboot
or shutdown. This is clearly not scalable as the number of
clients of SMMU would vary across SoCs and we would need to
add shutdown callbacks to almost all drivers eventually.
This callback was added for kexec usecase where it was known
to cause memory corruptions when SMMU was not shutdown but
that does not directly relate to SMMU because the memory
corruption could be because of the client of SMMU which is
not shutdown properly before booting into new kernel. So in
that case, we need to identify the client of SMMU causing
the memory corruption and add appropriate shutdown callback
to the client rather than to the SMMU.
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx>
---
drivers/iommu/arm-smmu-v3.c | 6 ------
drivers/iommu/arm-smmu.c | 6 ------
2 files changed, 12 deletions(-)
This feels like a giant bodge to me and I think that any driver which
continues to perform DMA after its ->shutdown() function has been
invoked
is buggy. Wouldn't that cause problems with kexec(), for example?
Yes it is definitely a bug in the client driver if DMA is performed
after shutdown callback of that respective driver is invoked and it
must
be fixed in that driver. But here the problem I was describing is not
that,
most of the drivers do not have a shutdown callback to begin with and
adding
it just because of shutdown dependency on SMMU doesn't seem so well
because
we can have many more such clients in the future and then we have to
just go
on adding the shutdown callbacks everywhere.
Yes, indeed we do. Like it or not, kexec is a thing, and about 98% of
drivers will have been written without considering it.
If fixing one problem exposes lots of other problems, can you honestly
say that the best solution is to go back and re-break the original
thing?
No, definitely not. I was under the wrong impression that *kexec thing*
was more due to the client driver bugs rather than the SMMU.
There's a clear shutdown dependency ordering, where the clients of
the
SMMU need to shutdown before the SMMU itself, but that's not really
the SMMU driver's problem to solve.
The problem with kexec may not be directly related to SMMU as you said
above if DMA is performed after the client shutdown callback, then its
a
bug in the client driver, so that needs to be fixed in the client
driver,
not the SMMU. So is there any point in having the SMMU shutdown
callback?
The point is that kexec needs to return the system to a state as close
to reset as possible. The SMMU is at least as relevant as any other
device in that regard, if not far more so - consider if the first
kernel *did* leave it enabled with whatever left-over translations in
place, and kexec'ed into another OS that wasn't SMMU-aware...
Yes understood. I wrongly assumed that the kexec problem was more
of a client driver bugs rather than SMMU. But I see that we indeed
need to shutdown SMMU as any other driver.
As you see, with this SMMU shutdown callback, we need to add shutdown
callbacks in all the client drivers.
Yes. And if you really want to argue against that, then at least be
consistent and propose removing shutdown from *all* the IOMMU drivers.
And then probably propose removing kexec support from all their
respective architectures... ;)
Not my intention to break or remove kexec, hence the RFC tag :)
As for the consistent part, out of 18 iommu drivers only 5
have shutdown callbacks, so nothing much to remove there and
kexec is broken there probably. However I got your point and
will drop this patch.
Thanks,
Sai
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