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Subject: Re: [PATCH 1/2] powerpc/64s: remove PROT_SAO support
To: Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org
References: <20200607120209.463501-1-npiggin@gmail.com>
From: Shawn Anastasio <shawn@anastas.io>
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Date: Mon, 17 Aug 2020 14:14:27 -0500
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

I'm a bit concerned about the removal of PROT_SAO.

 From what I can see, a feature like this would be extremely useful for
emulating architectures with stronger memory models. QEMU's multi-
threaded TCG project in particular looks like it would be a good
candidate, since as far as I'm aware it is currently completely
unable to perform strong-on-weak emulation.

Without hardware support like SAO provides, the only way I could see
to achieve this would be by emitting tons of unnecessary and costly
memory barrier instructions.

I understand that ISA 3.1 and POWER10 have dropped SAO, but as a POWER9
user it seems a bit silly to have a potentially useful feature dropped
from the kernel just because a future processor doesn't support it.

Curious to hear more thoughts on this.

Regards,
Shawn

On 6/7/20 7:02 AM, Nicholas Piggin wrote:
> ISA v3.1 does not support the SAO storage control attribute required to
> implement PROT_SAO. PROT_SAO was used by specialised system software
> (Lx86) that has been discontinued for about 7 years, and is not thought
> to be used elsewhere, so removal should not cause problems.
> 
> We rather remove it than keep support for older processors, because
> live migrating guest partitions to newer processors may not be possible
> if SAO is in use.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   arch/powerpc/include/asm/book3s/64/pgtable.h  |  8 ++--
>   arch/powerpc/include/asm/cputable.h           |  9 ++--
>   arch/powerpc/include/asm/kvm_book3s_64.h      |  3 +-
>   arch/powerpc/include/asm/mman.h               | 24 +++--------
>   arch/powerpc/include/asm/nohash/64/pgtable.h  |  2 -
>   arch/powerpc/kernel/dt_cpu_ftrs.c             |  2 +-
>   arch/powerpc/mm/book3s64/hash_utils.c         |  2 -
>   include/linux/mm.h                            |  2 -
>   include/trace/events/mmflags.h                |  2 -
>   mm/ksm.c                                      |  4 --
>   tools/testing/selftests/powerpc/mm/.gitignore |  1 -
>   tools/testing/selftests/powerpc/mm/Makefile   |  4 +-
>   tools/testing/selftests/powerpc/mm/prot_sao.c | 42 -------------------
>   13 files changed, 18 insertions(+), 87 deletions(-)
>   delete mode 100644 tools/testing/selftests/powerpc/mm/prot_sao.c
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index f17442c3a092..d9e92586f8dc 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -20,9 +20,13 @@
>   #define _PAGE_RW		(_PAGE_READ | _PAGE_WRITE)
>   #define _PAGE_RWX		(_PAGE_READ | _PAGE_WRITE | _PAGE_EXEC)
>   #define _PAGE_PRIVILEGED	0x00008 /* kernel access only */
> -#define _PAGE_SAO		0x00010 /* Strong access order */
> +
> +#define _PAGE_CACHE_CTL		0x00030 /* Bits for the folowing cache modes */
> +			/*	No bits set is normal cacheable memory */
> +			/*	0x00010 unused, is SAO bit on radix POWER9 */
>   #define _PAGE_NON_IDEMPOTENT	0x00020 /* non idempotent memory */
>   #define _PAGE_TOLERANT		0x00030 /* tolerant memory, cache inhibited */
> +
>   #define _PAGE_DIRTY		0x00080 /* C: page changed */
>   #define _PAGE_ACCESSED		0x00100 /* R: page referenced */
>   /*
> @@ -825,8 +829,6 @@ static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
>   	return hash__set_pte_at(mm, addr, ptep, pte, percpu);
>   }
>   
> -#define _PAGE_CACHE_CTL	(_PAGE_SAO | _PAGE_NON_IDEMPOTENT | _PAGE_TOLERANT)
> -
>   #define pgprot_noncached pgprot_noncached
>   static inline pgprot_t pgprot_noncached(pgprot_t prot)
>   {
> diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
> index bac2252c839e..c7e923b0000a 100644
> --- a/arch/powerpc/include/asm/cputable.h
> +++ b/arch/powerpc/include/asm/cputable.h
> @@ -191,7 +191,6 @@ static inline void cpu_feature_keys_init(void) { }
>   #define CPU_FTR_SPURR			LONG_ASM_CONST(0x0000000001000000)
>   #define CPU_FTR_DSCR			LONG_ASM_CONST(0x0000000002000000)
>   #define CPU_FTR_VSX			LONG_ASM_CONST(0x0000000004000000)
> -#define CPU_FTR_SAO			LONG_ASM_CONST(0x0000000008000000)
>   #define CPU_FTR_CP_USE_DCBTZ		LONG_ASM_CONST(0x0000000010000000)
>   #define CPU_FTR_UNALIGNED_LD_STD	LONG_ASM_CONST(0x0000000020000000)
>   #define CPU_FTR_ASYM_SMT		LONG_ASM_CONST(0x0000000040000000)
> @@ -435,7 +434,7 @@ static inline void cpu_feature_keys_init(void) { }
>   	    CPU_FTR_MMCRA | CPU_FTR_SMT | \
>   	    CPU_FTR_COHERENT_ICACHE | \
>   	    CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
> -	    CPU_FTR_DSCR | CPU_FTR_SAO  | CPU_FTR_ASYM_SMT | \
> +	    CPU_FTR_DSCR | CPU_FTR_ASYM_SMT | \
>   	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_POPCNTD | \
>   	    CPU_FTR_CFAR | CPU_FTR_HVMODE | \
>   	    CPU_FTR_VMX_COPY | CPU_FTR_HAS_PPR | CPU_FTR_DABRX | CPU_FTR_PKEY)
> @@ -444,7 +443,7 @@ static inline void cpu_feature_keys_init(void) { }
>   	    CPU_FTR_MMCRA | CPU_FTR_SMT | \
>   	    CPU_FTR_COHERENT_ICACHE | \
>   	    CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
> -	    CPU_FTR_DSCR | CPU_FTR_SAO  | \
> +	    CPU_FTR_DSCR | \
>   	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_POPCNTD | \
>   	    CPU_FTR_CFAR | CPU_FTR_HVMODE | CPU_FTR_VMX_COPY | \
>   	    CPU_FTR_DBELL | CPU_FTR_HAS_PPR | CPU_FTR_DAWR | \
> @@ -455,7 +454,7 @@ static inline void cpu_feature_keys_init(void) { }
>   	    CPU_FTR_MMCRA | CPU_FTR_SMT | \
>   	    CPU_FTR_COHERENT_ICACHE | \
>   	    CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
> -	    CPU_FTR_DSCR | CPU_FTR_SAO  | \
> +	    CPU_FTR_DSCR | \
>   	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_POPCNTD | \
>   	    CPU_FTR_CFAR | CPU_FTR_HVMODE | CPU_FTR_VMX_COPY | \
>   	    CPU_FTR_DBELL | CPU_FTR_HAS_PPR | CPU_FTR_ARCH_207S | \
> @@ -473,7 +472,7 @@ static inline void cpu_feature_keys_init(void) { }
>   	    CPU_FTR_MMCRA | CPU_FTR_SMT | \
>   	    CPU_FTR_COHERENT_ICACHE | \
>   	    CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
> -	    CPU_FTR_DSCR | CPU_FTR_SAO  | \
> +	    CPU_FTR_DSCR | \
>   	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_POPCNTD | \
>   	    CPU_FTR_CFAR | CPU_FTR_HVMODE | CPU_FTR_VMX_COPY | \
>   	    CPU_FTR_DBELL | CPU_FTR_HAS_PPR | CPU_FTR_ARCH_207S | \
> diff --git a/arch/powerpc/include/asm/kvm_book3s_64.h b/arch/powerpc/include/asm/kvm_book3s_64.h
> index 9bb9bb370b53..579c9229124b 100644
> --- a/arch/powerpc/include/asm/kvm_book3s_64.h
> +++ b/arch/powerpc/include/asm/kvm_book3s_64.h
> @@ -400,7 +400,8 @@ static inline bool hpte_cache_flags_ok(unsigned long hptel, bool is_ci)
>   
>   	/* Handle SAO */
>   	if (wimg == (HPTE_R_W | HPTE_R_I | HPTE_R_M) &&
> -	    cpu_has_feature(CPU_FTR_ARCH_206))
> +	    cpu_has_feature(CPU_FTR_ARCH_206) &&
> +	    !cpu_has_feature(CPU_FTR_ARCH_31))
>   		wimg = HPTE_R_M;
>   
>   	if (!is_ci)
> diff --git a/arch/powerpc/include/asm/mman.h b/arch/powerpc/include/asm/mman.h
> index d610c2e07b28..43a62f3e21a0 100644
> --- a/arch/powerpc/include/asm/mman.h
> +++ b/arch/powerpc/include/asm/mman.h
> @@ -13,38 +13,24 @@
>   #include <linux/pkeys.h>
>   #include <asm/cpu_has_feature.h>
>   
> -/*
> - * This file is included by linux/mman.h, so we can't use cacl_vm_prot_bits()
> - * here.  How important is the optimization?
> - */
> +#ifdef CONFIG_PPC_MEM_KEYS
>   static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
>   		unsigned long pkey)
>   {
> -#ifdef CONFIG_PPC_MEM_KEYS
> -	return (((prot & PROT_SAO) ? VM_SAO : 0) | pkey_to_vmflag_bits(pkey));
> -#else
> -	return ((prot & PROT_SAO) ? VM_SAO : 0);
> -#endif
> +	return pkey_to_vmflag_bits(pkey);
>   }
>   #define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
>   
>   static inline pgprot_t arch_vm_get_page_prot(unsigned long vm_flags)
>   {
> -#ifdef CONFIG_PPC_MEM_KEYS
> -	return (vm_flags & VM_SAO) ?
> -		__pgprot(_PAGE_SAO | vmflag_to_pte_pkey_bits(vm_flags)) :
> -		__pgprot(0 | vmflag_to_pte_pkey_bits(vm_flags));
> -#else
> -	return (vm_flags & VM_SAO) ? __pgprot(_PAGE_SAO) : __pgprot(0);
> -#endif
> +	return __pgprot(vmflag_to_pte_pkey_bits(vm_flags));
>   }
>   #define arch_vm_get_page_prot(vm_flags) arch_vm_get_page_prot(vm_flags)
> +#endif
>   
>   static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
>   {
> -	if (prot & ~(PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM | PROT_SAO))
> -		return false;
> -	if ((prot & PROT_SAO) && !cpu_has_feature(CPU_FTR_SAO))
> +	if (prot & ~(PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM))
>   		return false;
>   	return true;
>   }
> diff --git a/arch/powerpc/include/asm/nohash/64/pgtable.h b/arch/powerpc/include/asm/nohash/64/pgtable.h
> index 3424381b81da..2fd528ef48e0 100644
> --- a/arch/powerpc/include/asm/nohash/64/pgtable.h
> +++ b/arch/powerpc/include/asm/nohash/64/pgtable.h
> @@ -82,8 +82,6 @@
>    */
>   #include <asm/nohash/pte-book3e.h>
>   
> -#define _PAGE_SAO	0
> -
>   #define PTE_RPN_MASK	(~((1UL << PTE_RPN_SHIFT) - 1))
>   
>   /*
> diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
> index 3a409517c031..8d2e4043702f 100644
> --- a/arch/powerpc/kernel/dt_cpu_ftrs.c
> +++ b/arch/powerpc/kernel/dt_cpu_ftrs.c
> @@ -622,7 +622,7 @@ static struct dt_cpu_feature_match __initdata
>   	{"processor-control-facility-v3", feat_enable_dbell, CPU_FTR_DBELL},
>   	{"processor-utilization-of-resources-register", feat_enable_purr, 0},
>   	{"no-execute", feat_enable, 0},
> -	{"strong-access-ordering", feat_enable, CPU_FTR_SAO},
> +	{"strong-access-ordering", feat_enable, 0},
>   	{"cache-inhibited-large-page", feat_enable_large_ci, 0},
>   	{"coprocessor-icswx", feat_enable, 0},
>   	{"hypervisor-virtualization-interrupt", feat_enable_hvi, 0},
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index 0124003e60d0..14b6abdc3bd8 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -232,8 +232,6 @@ unsigned long htab_convert_pte_flags(unsigned long pteflags)
>   		rflags |= HPTE_R_I;
>   	else if ((pteflags & _PAGE_CACHE_CTL) == _PAGE_NON_IDEMPOTENT)
>   		rflags |= (HPTE_R_I | HPTE_R_G);
> -	else if ((pteflags & _PAGE_CACHE_CTL) == _PAGE_SAO)
> -		rflags |= (HPTE_R_W | HPTE_R_I | HPTE_R_M);
>   	else
>   		/*
>   		 * Add memory coherence if cache inhibited is not set
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 86adc71a972f..bdcaae914120 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -316,8 +316,6 @@ extern unsigned int kobjsize(const void *objp);
>   
>   #if defined(CONFIG_X86)
>   # define VM_PAT		VM_ARCH_1	/* PAT reserves whole VMA at once (x86) */
> -#elif defined(CONFIG_PPC)
> -# define VM_SAO		VM_ARCH_1	/* Strong Access Ordering (powerpc) */
>   #elif defined(CONFIG_PARISC)
>   # define VM_GROWSUP	VM_ARCH_1
>   #elif defined(CONFIG_IA64)
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 5fb752034386..939092dbcb8b 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -114,8 +114,6 @@ IF_HAVE_PG_IDLE(PG_idle,		"idle"		)
>   
>   #if defined(CONFIG_X86)
>   #define __VM_ARCH_SPECIFIC_1 {VM_PAT,     "pat"           }
> -#elif defined(CONFIG_PPC)
> -#define __VM_ARCH_SPECIFIC_1 {VM_SAO,     "sao"           }
>   #elif defined(CONFIG_PARISC) || defined(CONFIG_IA64)
>   #define __VM_ARCH_SPECIFIC_1 {VM_GROWSUP,	"growsup"	}
>   #elif !defined(CONFIG_MMU)
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 18c5d005bd01..b225b0e16111 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2452,10 +2452,6 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
>   		if (vma_is_dax(vma))
>   			return 0;
>   
> -#ifdef VM_SAO
> -		if (*vm_flags & VM_SAO)
> -			return 0;
> -#endif
>   #ifdef VM_SPARC_ADI
>   		if (*vm_flags & VM_SPARC_ADI)
>   			return 0;
> diff --git a/tools/testing/selftests/powerpc/mm/.gitignore b/tools/testing/selftests/powerpc/mm/.gitignore
> index 2ca523255b1b..ff296c94f627 100644
> --- a/tools/testing/selftests/powerpc/mm/.gitignore
> +++ b/tools/testing/selftests/powerpc/mm/.gitignore
> @@ -2,7 +2,6 @@
>   hugetlb_vs_thp_test
>   subpage_prot
>   tempfile
> -prot_sao
>   segv_errors
>   wild_bctr
>   large_vm_fork_separation
> diff --git a/tools/testing/selftests/powerpc/mm/Makefile b/tools/testing/selftests/powerpc/mm/Makefile
> index b9103c4bb414..9b8a7b3069c5 100644
> --- a/tools/testing/selftests/powerpc/mm/Makefile
> +++ b/tools/testing/selftests/powerpc/mm/Makefile
> @@ -2,7 +2,7 @@
>   noarg:
>   	$(MAKE) -C ../
>   
> -TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors wild_bctr \
> +TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot segv_errors wild_bctr \
>   		  large_vm_fork_separation bad_accesses
>   TEST_GEN_PROGS_EXTENDED := tlbie_test
>   TEST_GEN_FILES := tempfile
> @@ -12,8 +12,6 @@ include ../../lib.mk
>   
>   $(TEST_GEN_PROGS): ../harness.c
>   
> -$(OUTPUT)/prot_sao: ../utils.c
> -
>   $(OUTPUT)/wild_bctr: CFLAGS += -m64
>   $(OUTPUT)/large_vm_fork_separation: CFLAGS += -m64
>   $(OUTPUT)/bad_accesses: CFLAGS += -m64
> diff --git a/tools/testing/selftests/powerpc/mm/prot_sao.c b/tools/testing/selftests/powerpc/mm/prot_sao.c
> deleted file mode 100644
> index e2eed65b7735..000000000000
> --- a/tools/testing/selftests/powerpc/mm/prot_sao.c
> +++ /dev/null
> @@ -1,42 +0,0 @@
> -// SPDX-License-Identifier: GPL-2.0-only
> -/*
> - * Copyright 2016, Michael Ellerman, IBM Corp.
> - */
> -
> -#include <stdio.h>
> -#include <stdlib.h>
> -#include <string.h>
> -#include <sys/mman.h>
> -
> -#include <asm/cputable.h>
> -
> -#include "utils.h"
> -
> -#define SIZE (64 * 1024)
> -
> -int test_prot_sao(void)
> -{
> -	char *p;
> -
> -	/* 2.06 or later should support SAO */
> -	SKIP_IF(!have_hwcap(PPC_FEATURE_ARCH_2_06));
> -
> -	/*
> -	 * Ensure we can ask for PROT_SAO.
> -	 * We can't really verify that it does the right thing, but at least we
> -	 * confirm the kernel will accept it.
> -	 */
> -	p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE | PROT_SAO,
> -		 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> -	FAIL_IF(p == MAP_FAILED);
> -
> -	/* Write to the mapping, to at least cause a fault */
> -	memset(p, 0xaa, SIZE);
> -
> -	return 0;
> -}
> -
> -int main(void)
> -{
> -	return test_harness(test_prot_sao, "prot-sao");
> -}
>