Re: [RFC PATCH] arm64: mm: Add invalidate back in arch_sync_dma_for_device when FROM_DEVICE
From: Nanyong Sun
Date: Tue Dec 06 2022 - 12:08:47 EST
Hi Will,
What do you think about this?
I have another question: are there actual programs or scenarios
which can utilize the 2 problems fixed by the
commit c50f11c6196f ("arm64: mm: Don't invalidate FROM_DEVICE buffers at
start of DMA transfer")?
We want to evaluate the actual impact of this pach.
On 2022/11/25 4:04, Robin Murphy wrote:
On 2022-11-20 02:59, Nanyong Sun wrote:
On 2022/11/17 16:24, Ard Biesheuvel wrote:
On Thu, 17 Nov 2022 at 07:33, Nanyong Sun <sunnanyong@xxxxxxxxxx>
wrote:
The commit c50f11c6196f ("arm64: mm: Don't invalidate FROM_DEVICE
buffers at start of DMA transfer") replaces invalidate with clean
when DMA_FROM_DEVICE, this changes the behavior of functions like
dma_map_single() and dma_sync_single_for_device(*, *, *,
DMA_FROM_DEVICE),
then it may make some drivers works unwell because the implementation
of these DMA APIs lose the original cache invalidation.
Situation 1:
...
Situation 2:
After backporting the above commit, we find a network card driver go
wrong with cache inconsistency when doing DMA transfer: CPU got the
stale data in cache when reading DMA data received from device.
I suppose this means those drivers may lack dma_sync_single_for_cpu()
calls after the inbound transfers complete, and are instead relying on
the cache invalidation performed before the transfer to make the DMA'd
data visible to the CPU.
This is buggy and fragile, and should be fixed in any case. There is
no guarantee that the CPU will not preload parts of the buffer into
the cache while DMA is in progress, so the invalidation must occur
strictly after the device has finished transferring the data.
A similar phenomenon happens on sata disk drivers, it involves
mainline modules like libata, scsi, ahci etc, and is hard to find
out which line of code results in the error.
Could you identify the actual hardware and drivers that you are
observing the issue on? Claiming that everything is broken is not very
helpful in narrowing it down (although I am not saying you are wrong
:-))
The hardware combination is ARM64 with SATA disk，but not every
product of
this combination has the DMA problem, and the related drivers seems
right,
so I am currently checking whether the DT or ACPI indicate device's
coherent attribute uncorrectly.
(The scenario that Robin proposed in another email:
===============================================================
It also commonly goes wrong the other way round when the drivers are
correct but DT/ACPI failed to indicate a coherent device as such.
If writes from the device actually snoop, they hit the still-present
cache line, which then gets invalidated by unmap/sync_for_cpu and the
new data is lost.
Robin.
===============================================================
)
It seems that some dirvers may go wrong and have to match the
implementation changes of the DMA APIs, and it would be confused
because the behavior of these DMA APIs on arm64 are different
from other archs.
Add invalidate back in arch_sync_dma_for_device() to keep drivers
compatible by replace dcache_clean_poc with dcache_clean_inval_poc
when DMA_FROM_DEVICE.
So notably, the patch in question removes cache invalidation *without*
clean, and what you are adding here is clean+invalidate. (Invalidation
without clean may undo the effect of, e.g., the memzero() of a secret
in memory, and so it is important that we don't add that back if we
can avoid it)
Since we won't lose the benefits of that change, incorporating
invalidation at this point should be fine: clean+invalidate shouldn't
be more expensive than clean, and [correctly written] drivers will
invalidate those lines anyway, as the data has to come from DRAM in
any case.
So unless fixing the drivers in question is feasible, this change
seems reasonable to me.
Agree with you and I have some questions:
1. I am not very clear that how to fix the drivers? Before the patch
in question, the behaviors of DMA APIs are like this:
map for_cpu for_device unmap
TO_DEV writeback none writeback none
TO_CPU invalidate invalidate* invalidate invalidate*
BIDIR writeback invalidate writeback invalidate
(Reference from:
https://lore.kernel.org/lkml/20180518175004.GF17671@xxxxxxxxxxxxxxxxxxxxx/)
and now the behaviors on arm64 become this:
map for_cpu for_device
unmap
TO_DEV writeback none writeback none
TO_CPU -> [writeback] invalidate* ->[writeback] invalidate*
BIDIR writeback invalidate writeback invalidate
Can we confirm that these changes are acceptable on the ARM64?
I counted the drivers on the Linux mainline and there are at least
123 places of code have called
dma_sync_single_for_device(*, *, *, DMA_FROM_DEVICE), one of them
like this:
drivers/net/ethernet/sun/cassini.c:
cas_rx_process_pkt():
dma_sync_single_for_cpu(&cp->pdev->dev, page->dma_addr + off,
i, DMA_FROM_DEVICE);
addr = cas_page_map(page->buffer);
memcpy(p, addr + off, i);
dma_sync_single_for_device(&cp->pdev->dev,
page->dma_addr + off, i,
DMA_FROM_DEVICE);
Are they correct?
Yes.
In the non-coherent scenario here, dma_sync_single_for_cpu()
invalidates any previously-fetched cachelines to ensure that the
latest data written to DRAM by the device is visible to the CPU. There
is no writeback because those cachelines can only be clean, not dirty
- the device is non-coherent, so is not updating the cache, and the
contract of the DMA API says that nothing else may write to the same
address at the same time. The subsequent dma_sync_single_for_device()
doesn't actually need to do anything, because the whole transfer is
DMA_FROM_DEVICE - even though the buffer was almost certainly fetched
into some level of cache by the memcpy() reading from it, that's fine.
The device will write the next packet to DRAM, and those (clean)
cachelines will be invalidated by dma_sync_single_for_cpu() when we
start the whole cycle again.
So all of the dma_sync_single_for_device(*, *, *, DMA_FROM_DEVICE) are
unnecessary?
- If they are, they may be affected by the implementation change,
then how to fix them?
- Or these codes are needless on arm64 so they won't be affected?
2. Finding the drivers which do not strictly follow the DMA API rule
is expensive, they used to run with no problem
because they called the DMA APIs that will do the same invalidate
thing like dma_sync_single_for_cpu(), but now they have risks,
so every DMA users should check the drivers, including in tree,
out-of-tree and binary-only drivers.
We also need to check the DTS and ACPI to prevent the case that Robin
mentioned.
And are there any other scenarios we haven't thought of that need to
be checked?
That will be a huge and difficult workload for DMA users, and based
on they know the influence of that patch.
So, adding invalidate back in arch_sync_dma_for_device() seems more
friendly, cheap and harmless, the behaviors on arm64 will be this:
map for_cpu for_device
unmap
TO_DEV writeback none writeback none
TO_CPU wback+inv invalidate* wback+inv invalidate*
BIDIR writeback invalidate writeback invalidate
But then DMA_BIDIRECTIONAL is still wrong, and even if you bodge the
syncs even further to make that appear to work on an
incorrectly-configured system, dma_alloc_coherent() will still be at
risk of losing coherency in ways that cannot be
But only DMA_FROM_DEVICE is affected, if something wrong with
DMA_BIDIRECTIONAL, it would appear early before.
Why dma_alloc_coherent() will be at risk of losing coherency?
fixed at all, except by fixing the firmware to remove the source of
the problem.
In practice, driver bugs tend to be fairly easy to weed out with
options like CONFIG_DMA_API_DEBUG and "swiotlb=force", and given that
we can't reliably or completely work around broken firmware, I'd argue
that it's better *not* to try to hide these issues, to increase the
likelihood that they'll be noticed by developers earlier. As happened
again today, in fact:
I am afraid that it is not so easy in practice, at least we have to try
every dma-related hardware component of every product, and check the
drivers, DTS/ACPI.
https://lore.kernel.org/linux-arm-kernel/20221124142501.29314-1-johan+linaro@xxxxxxxxxx/
Thanks,
Robin.
Fixes: c50f11c6196f ("arm64: mm: Don't invalidate FROM_DEVICE
buffers at start of DMA transfer")
Signed-off-by: Nanyong Sun <sunnanyong@xxxxxxxxxx>
---
arch/arm64/mm/dma-mapping.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
index 3cb101e8cb29..07f6a3089c64 100644
--- a/arch/arm64/mm/dma-mapping.c
+++ b/arch/arm64/mm/dma-mapping.c
@@ -18,7 +18,10 @@ void arch_sync_dma_for_device(phys_addr_t paddr,
size_t size,
{
unsigned long start = (unsigned long)phys_to_virt(paddr);
- dcache_clean_poc(start, start + size);
+ if (dir == DMA_FROM_DEVICE)
+ dcache_clean_inval_poc(start, start + size);
+ else
+ dcache_clean_poc(start, start + size);
}
void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
--
2.25.1
.
.