Revert "iommu/dma: Add config for PCI SAC address trick"

This reverts commit 4bf7fda.

It turns out that it was hopelessly naive to think that this would work,
considering that we've always done this.  The first machine I actually
tested this on broke at bootup, getting to

    Reached target cryptsetup.target - Local Encrypted Volumes.

and then hanging.  It's unclear what actually fails, since there's a lot
else going on around that time (eg amdgpu probing also happens around
that same time, but it could be some other random init thing that didn't
complete earlier and just caused the boot to hang at that point).

The expectations that we should default to some unsafe and untested mode
seems entirely unfounded, and the belief that this wouldn't affect
modern systems is clearly entirely false.  The machine in question is
about two years old, so it's not exactly shiny, but it's also not some
dusty old museum piece PDP-11 in a closet.

Cc: Robin Murphy <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: John Garry <[email protected]>
Cc: Joerg Roedel <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/iommu/Kconfig

@@ -144,32 +144,6 @@ config IOMMU_DMA
 	select IRQ_MSI_IOMMU
 	select NEED_SG_DMA_LENGTH
 
-config IOMMU_DMA_PCI_SAC
-	bool "Enable 64-bit legacy PCI optimisation by default"
-	depends on IOMMU_DMA
-	help
-	  Enable by default an IOMMU optimisation for 64-bit legacy PCI devices,
-	  wherein the DMA API layer will always first try to allocate a 32-bit
-	  DMA address suitable for a single address cycle, before falling back
-	  to allocating from the device's full usable address range. If your
-	  system has 64-bit legacy PCI devices in 32-bit slots where using dual
-	  address cycles reduces DMA throughput significantly, this may be
-	  beneficial to overall performance.
-
-	  If you have a modern PCI Express based system, this feature mostly just
-	  represents extra overhead in the allocation path for no practical
-	  benefit, and it should usually be preferable to say "n" here.
-
-	  However, beware that this feature has also historically papered over
-	  bugs where the IOMMU address width and/or device DMA mask is not set
-	  correctly. If device DMA problems and IOMMU faults start occurring
-	  after disabling this option, it is almost certainly indicative of a
-	  latent driver or firmware/BIOS bug, which would previously have only
-	  manifested with several gigabytes worth of concurrent DMA mappings.
-
-	  If this option is not set, the feature can still be re-enabled at
-	  boot time with the "iommu.forcedac=0" command-line argument.
-
 # Shared Virtual Addressing
 config IOMMU_SVA
 	bool

drivers/iommu/dma-iommu.c

@@ -69,7 +69,7 @@ struct iommu_dma_cookie {
 };
 
 static DEFINE_STATIC_KEY_FALSE(iommu_deferred_attach_enabled);
-bool iommu_dma_forcedac __read_mostly = !IS_ENABLED(CONFIG_IOMMU_DMA_PCI_SAC);
+bool iommu_dma_forcedac __read_mostly;
 
 static int __init iommu_dma_forcedac_setup(char *str)
 {