Am 08.02.21 um 10:48 schrieb Daniel Vetter:
On Mon, Feb 08, 2021 at 10:37:19AM +0100, Christian König wrote:
Am 07.02.21 um 22:50 schrieb Daniel Vetter:
[SNIP]
Clarification - as far as I know there are no page fault handlers for kernel
mappings. And we are talking about kernel mappings here, right ? If there were
I could solve all those issues the same as I do for user mappings, by
invalidating all existing mappings in the kernel (both kmaps and ioreamps)and
insert dummy zero or ~0 filled page instead.
Also, I assume forcefully remapping the IO BAR to ~0 filled page would involve
ioremap API and it's not something that I think can be easily done according to
am answer i got to a related topic a few weeks ago
https://www.spinics.net/lists/linux-pci/msg103396.html (that was the only reply
i got)
mmiotrace can, but only for debug, and only on x86 platforms:
https://www.kernel.org/doc/html/latest/trace/mmiotrace.html
Should be feasible (but maybe not worth the effort) to extend this to
support fake unplug.
Mhm, interesting idea you guys brought up here.
We don't need a page fault for this to work, all we need to do is to insert
dummy PTEs into the kernels page table at the place where previously the
MMIO mapping has been.
Simply pte trick isn't enough, because we need:
- drop all writes silently
- all reads return 0xff
ptes can't do that themselves, we minimally need write protection and then
silently proceed on each write fault without restarting the instruction.
Better would be to only catch reads, but x86 doesn't do write-only pte
permissions afaik.
You are not thinking far enough :)
The dummy PTE is point to a dummy MMIO page which is just never used.
That hast the exact same properties than our removed MMIO space just
doesn't goes bananas when a new device is MMIO mapped into that and our
driver still tries to write there.
Regards,
Christian.
But ugh ...
Otoh validating an entire driver like amdgpu without such a trick
against 0xff reads is practically impossible. So maybe you need to add
this as one of the tasks here?
Or I could just for validation purposes return ~0 from all reg reads in the code
and ignore writes if drm_dev_unplugged, this could already easily validate a big
portion of the code flow under such scenario.
Hm yeah if your really wrap them all, that should work too. Since
iommappings have __iomem pointer type, as long as amdgpu is sparse
warning free, should be doable to guarantee this.
Problem is that ~0 is not always a valid register value.
You would need to audit every register read that it doesn't use the returned
value blindly as index or similar. That is quite a bit of work.
Yeah that's the entire crux here :-/
-Daniel
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