On 12/17/20 3:42 PM, Daniel Vetter wrote:
On Thu, Dec 17, 2020 at 8:19 PM Andrey Grodzovsky
<Andrey.Grodzovsky@xxxxxxx> wrote:
On 12/17/20 7:01 AM, Daniel Vetter wrote:
On Wed, Dec 16, 2020 at 07:20:02PM -0500, Andrey Grodzovsky wrote:
On 12/16/20 6:15 PM, Daniel Vetter wrote:
On Wed, Dec 16, 2020 at 7:26 PM Andrey Grodzovsky
<Andrey.Grodzovsky@xxxxxxx> wrote:
On 12/16/20 12:12 PM, Daniel Vetter wrote:
On Wed, Dec 16, 2020 at 5:18 PM Christian König
<christian.koenig@xxxxxxx> wrote:
Am 16.12.20 um 17:13 schrieb Andrey Grodzovsky:
On 12/16/20 9:21 AM, Daniel Vetter wrote:
On Wed, Dec 16, 2020 at 9:04 AM Christian König
<ckoenig.leichtzumerken@xxxxxxxxx> wrote:
Am 15.12.20 um 21:18 schrieb Andrey Grodzovsky:
[SNIP]
While we can't control user application accesses to the mapped
buffers explicitly and hence we use page fault rerouting
I am thinking that in this case we may be able to sprinkle
drm_dev_enter/exit in any such sensitive place were we might
CPU access a DMA buffer from the kernel ?
Yes, I fear we are going to need that.
Things like CPU page table updates, ring buffer accesses and FW
memcpy ? Is there other places ?
Puh, good question. I have no idea.
Another point is that at this point the driver shouldn't access any
such buffers as we are at the process finishing the device.
AFAIK there is no page fault mechanism for kernel mappings so I
don't think there is anything else to do ?
Well there is a page fault handler for kernel mappings, but that one
just prints the stack trace into the system log and calls BUG(); :)
Long story short we need to avoid any access to released pages after
unplug. No matter if it's from the kernel or userspace.
I was just about to start guarding with drm_dev_enter/exit CPU
accesses from kernel to GTT ot VRAM buffers but then i looked more in
the code
and seems like ttm_tt_unpopulate just deletes DMA mappings (for the
sake of device to main memory access). Kernel page table is not
touched
until last bo refcount is dropped and the bo is released
(ttm_bo_release->destroy->amdgpu_bo_destroy->amdgpu_bo_kunmap). This
is both
for GTT BOs maped to kernel by kmap (or vmap) and for VRAM BOs mapped
by ioremap. So as i see it, nothing will bad will happen after we
unpopulate a BO while we still try to use a kernel mapping for it,
system memory pages backing GTT BOs are still mapped and not freed and
for
VRAM BOs same is for the IO physical ranges mapped into the kernel
page table since iounmap wasn't called yet.
The problem is the system pages would be freed and if we kernel driver
still happily write to them we are pretty much busted because we write
to freed up memory.
OK, i see i missed ttm_tt_unpopulate->..->ttm_pool_free which will
release
the GTT BO pages. But then isn't there a problem in ttm_bo_release since
ttm_bo_cleanup_memtype_use which also leads to pages release comes
before bo->destroy which unmaps the pages from kernel page table ? Won't
we have end up writing to freed memory in this time interval ? Don't we
need to postpone pages freeing to after kernel page table unmapping ?
BOs are only destroyed when there is a guarantee that nobody is
accessing them any more.
The problem here is that the pages as well as the VRAM can be
immediately reused after the hotplug event.
Similar for vram, if this is actual hotunplug and then replug, there's
going to be a different device behind the same mmio bar range most
likely (the higher bridges all this have the same windows assigned),
No idea how this actually works but if we haven't called iounmap yet
doesn't it mean that those physical ranges that are still mapped into
page
table should be reserved and cannot be reused for another
device ? As a guess, maybe another subrange from the higher bridge's
total
range will be allocated.
Nope, the PCIe subsystem doesn't care about any ioremap still active for
a range when it is hotplugged.
and that's bad news if we keep using it for current drivers. So we
really have to point all these cpu ptes to some other place.
We can't just unmap it without syncing against any in kernel accesses
to those buffers
and since page faulting technique we use for user mapped buffers seems
to not be possible
for kernel mapped buffers I am not sure how to do it gracefully...
We could try to replace the kmap with a dummy page under the hood, but
that is extremely tricky.
Especially since BOs which are just 1 page in size could point to the
linear mapping directly.
I think it's just more work. Essentially
- convert as much as possible of the kernel mappings to vmap_local,
which Thomas Zimmermann is rolling out. That way a dma_resv_lock will
serve as a barrier, and ofc any new vmap needs to fail or hand out a
dummy mapping.
Read those patches. I am not sure how this helps with protecting
against accesses to released backing pages or IO physical ranges of BO
which is already mapped during the unplug event ?
By eliminating such users, and replacing them with local maps which
are strictly bound in how long they can exist (and hence we can
serialize against them finishing in our hotunplug code).
Not sure I see how serializing against BO map/unmap helps - our problem as
you described is that once
device is extracted and then something else quickly takes it's place in the
PCI topology
and gets assigned same physical IO ranges, then our driver will start accessing this
new device because our 'zombie' BOs are still pointing to those ranges.
Until your driver's remove callback is finished the ranges stay reserved.
The ranges stay reserved until unmapped which happens in bo->destroy
which for most internally allocated buffers is during sw_fini when last drm_put
is called.
If that's not the case, then hotunplug would be fundamentally impossible
ot handle correctly.
Of course all the mmio actions will time out, so it might take some time
to get through it all.
I found that PCI code provides pci_device_is_present function
we can use to avoid timeouts - it reads device vendor and checks if all 1s is
returned
or not. We can call it from within register accessors before trying read/write
drm_dev_enter/exit is a _lot_ less overhead, plus makes a lot stronger
guarantees for hotunplug ordering. Please use that one instead of
hand-rolling something which only mostly works for closing hotunplug
races. pciconfig access is really slow.
Another point regarding serializing - problem is that some of those BOs are
very long lived, take for example the HW command
ring buffer Christian mentioned before -
(amdgpu_ring_init->amdgpu_bo_create_kernel), it's life span
is basically for the entire time the device exists, it's destroyed only in
the SW fini stage (when last drm_dev
reference is dropped) and so should I grab it's dma_resv_lock from
amdgpu_pci_remove code and wait
for it to be unmapped before proceeding with the PCI remove code ? This can
take unbound time and that why I don't understand
how serializing will help.
Uh you need to untangle that. After hw cleanup is done no one is allowed
to touch that ringbuffer bo anymore from the kernel.
I would assume we are not allowed to touch it once we identified the device is
gone in order to minimize the chance of accidental writes to some other device
which might now
occupy those IO ranges ?
That's what
drm_dev_enter/exit guards are for. Like you say we cant wait for all sw
references to disappear.
Yes, didn't make sense to me why would we use vmap_local for internally
allocated buffers. I think we should also guard registers read/writes for the
same reason as above.
The vmap_local is for mappings done by other drivers, through the dma-buf
interface (where "other drivers" can include fbdev/fbcon, if you use the
generic helpers).
-Daniel
Ok, so I assumed that with vmap_local you were trying to solve the problem of
quick reinsertion
of another device into same MMIO range that my driver still points too but
actually are you trying to solve
the issue of exported dma buffers outliving the device ? For this we have
drm_device refcount in the GEM layer
i think.
That's completely different lifetime problems. Don't mix them up :-)
One problem is the hardware disappearing, and for that we _have_ to
guarantee timeliness, or otherwise the pci subsystem gets pissed
(since like you say, a new device might show up and need it's mmio
bars assigned to io ranges). The other is lifetim of the software
objects we use as interfaces, both from userspace and from other
kernel drivers. There we fundamentally can't enforce timely cleanup,
and have to resort to refcounting.
So regarding the second issue, as I mentioned above, don't we already use
drm_dev_get/put
for exported BOs ? Earlier in this discussion you mentioned that we are ok for
dma buffers since
we already have the refcounting at the GEM layer and the real life cycle problem
we have is the dma_fences
for which there is no drm_dev refcounting. Seems to me then that vmap_local is
superfluous because
of the recounting we already have for exported dma_bufs and for dma_fences it
won't help.
Andrey
We need both.
-Daniel
Andrey
Andrey
It doesn't
solve all your problems, but it's a tool to get there.
-Daniel
Andrey
- handle fbcon somehow. I think shutting it all down should work out.
- worst case keep the system backing storage around for shared dma-buf
until the other non-dynamic driver releases it. for vram we require
dynamic importers (and maybe it wasn't such a bright idea to allow
pinning of importer buffers, might need to revisit that).
Cheers, Daniel
Christian.
Andrey
-Daniel
Christian.
I loaded the driver with vm_update_mode=3
meaning all VM updates done using CPU and hasn't seen any OOPs after
removing the device. I guess i can test it more by allocating GTT and
VRAM BOs
and trying to read/write to them after device is removed.
Andrey
Regards,
Christian.
Andrey
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