Am 08.02.21 um 23:09 schrieb Andrey Grodzovsky:
On 2/8/21 4:37 AM, 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.spinics.net%2Flists%2Flinux-pci%2Fmsg103396.html&data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7Cb159d3ce264944486c8008d8cc15233a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637483738446813868%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=6eP0nhS%2BZwp1Y54CwfX8vaV3FTWbW8IylW5JFaf92pY%3D&reserved=0
(that was the only reply
i got)
mmiotrace can, but only for debug, and only on x86 platforms:
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.kernel.org%2Fdoc%2Fhtml%2Flatest%2Ftrace%2Fmmiotrace.html&data=04%7C01%7CAndrey.Grodzovsky%40amd.com%7Cb159d3ce264944486c8008d8cc15233a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637483738446813868%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=QBF9J%2BVIRUkUTTjvNoZR8NqFNt8CpHkcknH2qKX7dd8%3D&reserved=0
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.
But that exactly what Mathew from linux-mm says is not a trivial thing
to do, quote:
"
ioremap() is done through the vmalloc space. It would, in theory, be
possible to reprogram the page tables used for vmalloc to point to your
magic page. I don't think we have such a mechanism today, and there are
lots of problems with things like TLB flushes. It's probably going to
be harder than you think.
"
I haven't followed the full discussion, but I don't see much preventing
this.
All you need is a new ioremap_dummy() function which takes the old start
and length of the mapping.
Still a bit core and maybe even platform code, but rather useful I think.
Christian.
If you believe it's actually doable then it would be useful not only for
simulating device
unplugged situation with all MMIOs returning 0xff... but for actual handling of
driver accesses
to MMIO after device is gone and, we could then drop entirely this patch as
there would be no need
to guard against such accesses post device unplug.
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.
But ~0 is the value that will be returned for every read post device
unplug, regardless if it's valid or not, and we have to cope with
it then, no ?
Andrey
Regards,
Christian.
-Daniel
Andrey
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