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Date:   Fri, 16 Nov 2018 08:52:14 -0800 (PST)
Subject:     Re: [PATCH 2/2] arch: switch the default on ARCH_HAS_SG_CHAIN
In-Reply-To: <20181109090008.24150-3-hch@lst.de>
CC:     iommu@lists.linux-foundation.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org
From:   Palmer Dabbelt <palmer@sifive.com>
To:     Christoph Hellwig <hch@lst.de>
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On Fri, 09 Nov 2018 01:00:08 PST (-0800), Christoph Hellwig wrote:
> These days architectures are mostly out of the business of dealing with
> struct scatterlist at all, unless they have architecture specific iommu
> drivers.  Replace the ARCH_HAS_SG_CHAIN symbol with a ARCH_NO_SG_CHAIN
> one only enabled for architectures with horrible legacy iommu drivers
> like alpha and parisc, and conditionally for arm which wants to keep it
> disable for legacy platforms.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  .../features/io/sg-chain/arch-support.txt     | 33 -------------------
>  arch/alpha/Kconfig                            |  1 +
>  arch/arc/Kconfig                              |  1 -
>  arch/arm/Kconfig                              |  2 +-
>  arch/arm64/Kconfig                            |  1 -
>  arch/ia64/Kconfig                             |  1 -
>  arch/parisc/Kconfig                           |  1 +
>  arch/powerpc/Kconfig                          |  1 -
>  arch/s390/Kconfig                             |  1 -
>  arch/sparc/Kconfig                            |  1 -
>  arch/x86/Kconfig                              |  1 -
>  arch/xtensa/Kconfig                           |  1 -
>  include/linux/scatterlist.h                   |  6 ++--
>  lib/Kconfig                                   |  2 +-
>  lib/scatterlist.c                             |  2 +-
>  15 files changed, 8 insertions(+), 47 deletions(-)
>  delete mode 100644 Documentation/features/io/sg-chain/arch-support.txt

As far as I can tell, it looks like m68k, mips, and powerpc mention an IOMMU in 
their ports, don't set ARCH_HAS_SG_CHAIN, and with this patch set won't set 
ARCH_NO_SG_CHAIN.  The issue is that I'm not sure how to determine what 
constitutes a horrible legacy IOMMU, at least with respect to not being able to 
use scatterlist chaining.

I poked through the 68k stuff (sun3 and sun3x) and the only thing I see that's 
odd is that DVMA_PAGE_SHIFT is 13.  That matches PAGE_SHIFT for the subarch, 
though, so I don't think it's an issue.  It looks like the mention in MIPS is 
trivial, so that should be fine.  There's a lot in the PC port so I just 
glanced at it.  I assume it's OK, but I did see an explicit mention of SG in 
their generic IOMMU.  There's a lot of flavors here so I'm not sure how well 
they all work.

I don't really think that's enough for my reviewed-by tag, so feel free to give 
me some pointers if you're looking for one -- I'd be happy to lean about 
IOMMUs (particularly broken ones, as that might help prevent us from designing 
a broken one), I'm just not sure what I'm looking for.

I'm going to assume you're going to merge this and I'll drop the RISC-V patch 
that turns on ARCH_HAS_SG_CHAIN.

Thanks!

> diff --git a/Documentation/features/io/sg-chain/arch-support.txt b/Documentation/features/io/sg-chain/arch-support.txt
> deleted file mode 100644
> index 6554f0372c3f..000000000000
> --- a/Documentation/features/io/sg-chain/arch-support.txt
> +++ /dev/null
> @@ -1,33 +0,0 @@
> -#
> -# Feature name:          sg-chain
> -#         Kconfig:       ARCH_HAS_SG_CHAIN
> -#         description:   arch supports chained scatter-gather lists
> -#
> -    -----------------------
> -    |         arch |status|
> -    -----------------------
> -    |       alpha: | TODO |
> -    |         arc: |  ok  |
> -    |         arm: |  ok  |
> -    |       arm64: |  ok  |
> -    |         c6x: | TODO |
> -    |       h8300: | TODO |
> -    |     hexagon: | TODO |
> -    |        ia64: |  ok  |
> -    |        m68k: | TODO |
> -    |  microblaze: | TODO |
> -    |        mips: | TODO |
> -    |       nds32: | TODO |
> -    |       nios2: | TODO |
> -    |    openrisc: | TODO |
> -    |      parisc: | TODO |
> -    |     powerpc: |  ok  |
> -    |       riscv: | TODO |
> -    |        s390: |  ok  |
> -    |          sh: | TODO |
> -    |       sparc: |  ok  |
> -    |          um: | TODO |
> -    |   unicore32: | TODO |
> -    |         x86: |  ok  |
> -    |      xtensa: | TODO |
> -    -----------------------
> diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
> index 5b4f88363453..a7e748a46c18 100644
> --- a/arch/alpha/Kconfig
> +++ b/arch/alpha/Kconfig
> @@ -5,6 +5,7 @@ config ALPHA
>  	select ARCH_MIGHT_HAVE_PC_PARPORT
>  	select ARCH_MIGHT_HAVE_PC_SERIO
>  	select ARCH_NO_PREEMPT
> +	select ARCH_NO_SG_CHAIN
>  	select ARCH_USE_CMPXCHG_LOCKREF
>  	select HAVE_AOUT
>  	select HAVE_IDE
> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> index c9e2a1323536..fd48d698da29 100644
> --- a/arch/arc/Kconfig
> +++ b/arch/arc/Kconfig
> @@ -13,7 +13,6 @@ config ARC
>  	select ARCH_HAS_PTE_SPECIAL
>  	select ARCH_HAS_SYNC_DMA_FOR_CPU
>  	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
> -	select ARCH_HAS_SG_CHAIN
>  	select ARCH_SUPPORTS_ATOMIC_RMW if ARC_HAS_LLSC
>  	select BUILDTIME_EXTABLE_SORT
>  	select CLONE_BACKWARDS
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 91be74d8df65..199eb62230b0 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -19,6 +19,7 @@ config ARM
>  	select ARCH_HAVE_CUSTOM_GPIO_H
>  	select ARCH_HAS_GCOV_PROFILE_ALL
>  	select ARCH_MIGHT_HAVE_PC_PARPORT
> +	select ARCH_NO_SG_CHAIN if !ARM_HAS_SG_CHAIN
>  	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
>  	select ARCH_SUPPORTS_ATOMIC_RMW
> @@ -118,7 +119,6 @@ config ARM
>  	  <http://www.arm.linux.org.uk/>.
>
>  config ARM_HAS_SG_CHAIN
> -	select ARCH_HAS_SG_CHAIN
>  	bool
>
>  config ARM_DMA_USE_IOMMU
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 787d7850e064..4c851b3a7b7c 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -23,7 +23,6 @@ config ARM64
>  	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>  	select ARCH_HAS_PTE_SPECIAL
>  	select ARCH_HAS_SET_MEMORY
> -	select ARCH_HAS_SG_CHAIN
>  	select ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_HAS_STRICT_MODULE_RWX
>  	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
> diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
> index 36773def6920..d6f203658994 100644
> --- a/arch/ia64/Kconfig
> +++ b/arch/ia64/Kconfig
> @@ -29,7 +29,6 @@ config IA64
>  	select HAVE_MEMBLOCK_NODE_MAP
>  	select HAVE_VIRT_CPU_ACCOUNTING
>  	select ARCH_HAS_DMA_MARK_CLEAN
> -	select ARCH_HAS_SG_CHAIN
>  	select VIRT_TO_BUS
>  	select ARCH_DISCARD_MEMBLOCK
>  	select GENERIC_IRQ_PROBE
> diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
> index 92a339ee28b3..428ee50fc3db 100644
> --- a/arch/parisc/Kconfig
> +++ b/arch/parisc/Kconfig
> @@ -11,6 +11,7 @@ config PARISC
>  	select ARCH_HAS_ELF_RANDOMIZE
>  	select ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_HAS_UBSAN_SANITIZE_ALL
> +	select ARCH_NO_SG_CHAIN
>  	select ARCH_SUPPORTS_MEMORY_FAILURE
>  	select RTC_CLASS
>  	select RTC_DRV_GENERIC
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 8be31261aec8..4bc8edd83cee 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -138,7 +138,6 @@ config PPC
>  	select ARCH_HAS_PTE_SPECIAL
>  	select ARCH_HAS_MEMBARRIER_CALLBACKS
>  	select ARCH_HAS_SCALED_CPUTIME		if VIRT_CPU_ACCOUNTING_NATIVE && PPC64
> -	select ARCH_HAS_SG_CHAIN
>  	select ARCH_HAS_STRICT_KERNEL_RWX	if ((PPC_BOOK3S_64 || PPC32) && !RELOCATABLE && !HIBERNATION)
>  	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST
>  	select ARCH_HAS_UACCESS_FLUSHCACHE	if PPC64
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index 5173366af8f3..5624e8607054 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -73,7 +73,6 @@ config S390
>  	select ARCH_HAS_KCOV
>  	select ARCH_HAS_PTE_SPECIAL
>  	select ARCH_HAS_SET_MEMORY
> -	select ARCH_HAS_SG_CHAIN
>  	select ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_HAS_STRICT_MODULE_RWX
>  	select ARCH_HAS_UBSAN_SANITIZE_ALL
> diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
> index 490b2c95c212..8853b6ceae17 100644
> --- a/arch/sparc/Kconfig
> +++ b/arch/sparc/Kconfig
> @@ -40,7 +40,6 @@ config SPARC
>  	select MODULES_USE_ELF_RELA
>  	select ODD_RT_SIGACTION
>  	select OLD_SIGSUSPEND
> -	select ARCH_HAS_SG_CHAIN
>  	select CPU_NO_EFFICIENT_FFS
>  	select LOCKDEP_SMALL if LOCKDEP
>  	select NEED_DMA_MAP_STATE
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index ba7e3464ee92..6faab6f9b57e 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -66,7 +66,6 @@ config X86
>  	select ARCH_HAS_UACCESS_FLUSHCACHE	if X86_64
>  	select ARCH_HAS_UACCESS_MCSAFE		if X86_64 && X86_MCE
>  	select ARCH_HAS_SET_MEMORY
> -	select ARCH_HAS_SG_CHAIN
>  	select ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_HAS_STRICT_MODULE_RWX
>  	select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
> index d29b7365da8d..e3387919eb22 100644
> --- a/arch/xtensa/Kconfig
> +++ b/arch/xtensa/Kconfig
> @@ -1,7 +1,6 @@
>  # SPDX-License-Identifier: GPL-2.0
>  config XTENSA
>  	def_bool y
> -	select ARCH_HAS_SG_CHAIN
>  	select ARCH_HAS_SYNC_DMA_FOR_CPU
>  	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
>  	select ARCH_NO_COHERENT_DMA_MMAP if !MMU
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 093aa57120b0..b96f0d0b5b8f 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -324,10 +324,10 @@ size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
>   * Like SG_CHUNK_SIZE, but for archs that have sg chaining. This limit
>   * is totally arbitrary, a setting of 2048 will get you at least 8mb ios.
>   */
> -#ifdef CONFIG_ARCH_HAS_SG_CHAIN
> -#define SG_MAX_SEGMENTS	2048
> -#else
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
>  #define SG_MAX_SEGMENTS	SG_CHUNK_SIZE
> +#else
> +#define SG_MAX_SEGMENTS	2048
>  #endif
>
>  #ifdef CONFIG_SG_POOL
> diff --git a/lib/Kconfig b/lib/Kconfig
> index a9965f4af4dd..d5a5e2ebf286 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -577,7 +577,7 @@ config SG_POOL
>  # sg chaining option
>  #
>
> -config ARCH_HAS_SG_CHAIN
> +config ARCH_NO_SG_CHAIN
>  	def_bool n
>
>  config ARCH_HAS_PMEM_API
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 7c6096a71704..9ba349e775ef 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -271,7 +271,7 @@ int __sg_alloc_table(struct sg_table *table, unsigned int nents,
>
>  	if (nents == 0)
>  		return -EINVAL;
> -#ifndef CONFIG_ARCH_HAS_SG_CHAIN
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
>  	if (WARN_ON_ONCE(nents > max_ents))
>  		return -EINVAL;
>  #endif