Re: [PATCH 15/21] ARM: dma-mapping: always invalidate WT caches before DMA

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On Fri, Mar 31, 2023, at 11:35, Russell King (Oracle) wrote:
On Fri, Mar 31, 2023 at 10:07:28AM +0100, Russell King (Oracle) wrote:
On Mon, Mar 27, 2023 at 02:13:11PM +0200, Arnd Bergmann wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>

Most ARM CPUs can have write-back caches and that require
cache management to be done in the dma_sync_*_for_device()
operation. This is typically done in both writeback and
writethrough mode.

The cache-v4.S (arm720/740/7tdmi/9tdmi) and cache-v4wt.S
(arm920t, arm940t) implementations are the exception here,
and only do the cache management after the DMA is complete,
in the dma_sync_*_for_cpu() operation.

Change this for consistency with the other platforms. This
should have no user visible effect.


The reason we do cache management _after_ is to ensure that there
is no stale data. The kernel _has_ (at the very least in the past)
performed DMA to data structures that are embedded within other
data structures, resulting in cache lines being shared. If one of
those cache lines is touched while DMA is progressing, then we
must to cache management _after_ the DMA operation has completed.
Doing it before is no good.

What I'm trying to address here is the inconsistency between
implementations. If we decide that we always want to invalidate
after FROM_DEVICE, I can do that as part of the series, but then
I have to change most of the other arm implementations.

Right now, the only WT cache implementations that do the the
invalidation after the DMA are cache-v4.S (arm720 integrator and
clps711x), cache-v4wt.S (arm920/arm922 at91rm9200, clps711x,
ep93xx, omap15xx, imx1 and integrator), some sparc32 leon3 and
early xtensa.

Most architectures that have write-through caches (m68k,
microblaze) or write-back caches but no speculation (all other
armv4/armv5, hexagon, openrisc, sh, most mips, later xtensa)
only invalidate before DMA but not after.

OTOH, most machines that are actually in use today (armv6+,
powerpc, later mips, microblaze, riscv, nios2) also have to
deal with speculative accesses, so they end up having to
invalidate or flush both before and after a DMA_FROM_DEVICE

It looks like the main offender of "touching cache lines shared
with DMA" has now been resolved - that was the SCSI sense buffer,
and was fixed some time ago:

commit de25deb18016f66dcdede165d07654559bb332bc
Author: FUJITA Tomonori <fujita.tomonori@xxxxxxxxxxxxx>
Date:   Wed Jan 16 13:32:17 2008 +0900

/if/ that is the one and only case, then we're probably fine, but
having been through an era where this kind of thing was the norm
and requests to fix it did not get great responses from subsystem
maintainers, I just don't trust the kernel not to want to DMA to
overlapping cache lines.

Thanks for digging that out, that is very useful. It looks like this
was around the same time as 03d70617b8a7 ("powerpc: Prevent memory
corruption due to cache invalidation of unaligned DMA buffer"), so
it may well have been related. I know we also had more recent 
problems with USB drivers trying to DMA to stack, which would 
also cause problems on non-coherent machines, but some of these were
only found after we introduced VMAP_STACK.

It would be nice to use KASAN prevent reads on cache lines that
have in-flight DMA.


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