| Message ID | 20200421142603.3894-2-catalin.marinas@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: Memory Tagging Extension user-space support | expand |
On Tue, Apr 21, 2020 at 03:25:41PM +0100, Catalin Marinas wrote: > There are situations where we do not want to disable the whole block > based on a config option, only the alternative part while keeping the > first instruction. Improve the alternative_insn assembler macro to take > a 'first_insn' argument, default 0, to preserve the current behaviour. > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> > Cc: Will Deacon <will@kernel.org> > --- > arch/arm64/include/asm/alternative.h | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h > index 5e5dc05d63a0..67d7cc608336 100644 > --- a/arch/arm64/include/asm/alternative.h > +++ b/arch/arm64/include/asm/alternative.h > @@ -111,7 +111,11 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > .byte \alt_len > .endm > > -.macro alternative_insn insn1, insn2, cap, enable = 1 > +/* > + * Disable the whole block if enable == 0, unless first_insn == 1 in which > + * case insn1 will always be issued but without an alternative insn2. > + */ > +.macro alternative_insn insn1, insn2, cap, enable = 1, first_insn = 0 > .if \enable > 661: \insn1 > 662: .pushsection .altinstructions, "a" > @@ -122,6 +126,8 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > 664: .popsection > .org . - (664b-663b) + (662b-661b) > .org . - (662b-661b) + (664b-663b) > + .elseif \first_insn > + \insn1 This becomes quite unreadable at the invocation site, especially when invoked as "alternative_insn ..., 1". "... first_insn=1" is not much better either). I'm struggling to find non-trivial users of this that actually want the whole block to be deleted dependent on the config. Can we instead just always behave as if first_insn=1 instead? This this works intuitively as an alternative, not the current weird 3-way choice between insn1, insn2 and nothing at all. The only time that makes sense is when one of the insns is a branch that skips the block, but that's handled via the alternative_if macros instead. Behaving always like first_insn=1 provides an if-else that is statically optimised if the relevant feature is configured out, which I think is the only thing people are ever going to want. Maybe something depends on the current behaviour, but I can't see it so far... [...] Cheers ---Dave
Hi Dave, On Mon, Apr 27, 2020 at 05:57:37PM +0100, Dave P Martin wrote: > On Tue, Apr 21, 2020 at 03:25:41PM +0100, Catalin Marinas wrote: > > There are situations where we do not want to disable the whole block > > based on a config option, only the alternative part while keeping the > > first instruction. Improve the alternative_insn assembler macro to take > > a 'first_insn' argument, default 0, to preserve the current behaviour. > > > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> > > Cc: Will Deacon <will@kernel.org> > > --- > > arch/arm64/include/asm/alternative.h | 8 +++++++- > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h > > index 5e5dc05d63a0..67d7cc608336 100644 > > --- a/arch/arm64/include/asm/alternative.h > > +++ b/arch/arm64/include/asm/alternative.h > > @@ -111,7 +111,11 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > > .byte \alt_len > > .endm > > > > -.macro alternative_insn insn1, insn2, cap, enable = 1 > > +/* > > + * Disable the whole block if enable == 0, unless first_insn == 1 in which > > + * case insn1 will always be issued but without an alternative insn2. > > + */ > > +.macro alternative_insn insn1, insn2, cap, enable = 1, first_insn = 0 > > .if \enable > > 661: \insn1 > > 662: .pushsection .altinstructions, "a" > > @@ -122,6 +126,8 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > > 664: .popsection > > .org . - (664b-663b) + (662b-661b) > > .org . - (662b-661b) + (664b-663b) > > + .elseif \first_insn > > + \insn1 > > This becomes quite unreadable at the invocation site, especially when > invoked as "alternative_insn ..., 1". "... first_insn=1" is not much > better either). That I agree. The reason I didn't leave the alternative in place here is that if gas doesn't support MTE, it will fail to compile. I wanted to avoid the several #ifdef's. > I'm struggling to find non-trivial users of this that actually want the > whole block to be deleted dependent on the config. Some of the errata stuff like CONFIG_ARM64_REPEAT_TLBI ends up with unnecessary nops. Similarly for CONFIG_ARM64_UAO/PAN and maybe a few others (it's all additional nops). We also have a few errata workaround where we didn't bother with the config enable option at all. While this is C code + inline asm, I'd like to have a consistent behaviour of ALTERNATIVE between C and .S files. Now, given that some of them (like UAO/PAN) are on by default, it probably doesn't make any difference if we always keep the first block (non-alternative). We could add a new macro ALTERNATIVE_OR_NOP. > Can we instead just always behave as if first_insn=1 instead? This this > works intuitively as an alternative, not the current weird 3-way choice > between insn1, insn2 and nothing at all. The only time that makes sense > is when one of the insns is a branch that skips the block, but that's > handled via the alternative_if macros instead. > > Behaving always like first_insn=1 provides an if-else that is statically > optimised if the relevant feature is configured out, which I think is > the only thing people are ever going to want. > > Maybe something depends on the current behaviour, but I can't see it so > far... I'll give it a go in v4 and see how it looks. Another option would be an alternative_else which takes an enable argument. Thanks.
On Tue, Apr 28, 2020 at 12:43:54PM +0100, Catalin Marinas wrote: > Hi Dave, > > On Mon, Apr 27, 2020 at 05:57:37PM +0100, Dave P Martin wrote: > > On Tue, Apr 21, 2020 at 03:25:41PM +0100, Catalin Marinas wrote: > > > There are situations where we do not want to disable the whole block > > > based on a config option, only the alternative part while keeping the > > > first instruction. Improve the alternative_insn assembler macro to take > > > a 'first_insn' argument, default 0, to preserve the current behaviour. > > > > > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> > > > Cc: Will Deacon <will@kernel.org> > > > --- > > > arch/arm64/include/asm/alternative.h | 8 +++++++- > > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h > > > index 5e5dc05d63a0..67d7cc608336 100644 > > > --- a/arch/arm64/include/asm/alternative.h > > > +++ b/arch/arm64/include/asm/alternative.h > > > @@ -111,7 +111,11 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > > > .byte \alt_len > > > .endm > > > > > > -.macro alternative_insn insn1, insn2, cap, enable = 1 > > > +/* > > > + * Disable the whole block if enable == 0, unless first_insn == 1 in which > > > + * case insn1 will always be issued but without an alternative insn2. > > > + */ > > > +.macro alternative_insn insn1, insn2, cap, enable = 1, first_insn = 0 > > > .if \enable > > > 661: \insn1 > > > 662: .pushsection .altinstructions, "a" > > > @@ -122,6 +126,8 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > > > 664: .popsection > > > .org . - (664b-663b) + (662b-661b) > > > .org . - (662b-661b) + (664b-663b) > > > + .elseif \first_insn > > > + \insn1 > > > > This becomes quite unreadable at the invocation site, especially when > > invoked as "alternative_insn ..., 1". "... first_insn=1" is not much > > better either). > > That I agree. > > The reason I didn't leave the alternative in place here is that if gas > doesn't support MTE, it will fail to compile. I wanted to avoid the > several #ifdef's. We could solve that by synthesising the opcodes instead of relying on gas (as we do for other extensions). But I'd agree that's just pushing the problem around rather than solving it. It seems dumb to go to that trouble for a case where the affected insn isn't going to be emitted... > > I'm struggling to find non-trivial users of this that actually want the > > whole block to be deleted dependent on the config. > > Some of the errata stuff like CONFIG_ARM64_REPEAT_TLBI ends up with > unnecessary nops. Similarly for CONFIG_ARM64_UAO/PAN and maybe a few > others (it's all additional nops). We also have a few errata workaround > where we didn't bother with the config enable option at all. OK, looks like I may have missed some cases. There's a dense thicket of macros that call each other here, and I've not looked at it for a while ;) > While this is C code + inline asm, I'd like to have a consistent > behaviour of ALTERNATIVE between C and .S files. Now, given that some of > them (like UAO/PAN) are on by default, it probably doesn't make any > difference if we always keep the first block (non-alternative). > > We could add a new macro ALTERNATIVE_OR_NOP. alternative_insn doesn't seem exist for C at all. Did I miss something? > > Can we instead just always behave as if first_insn=1 instead? This this > > works intuitively as an alternative, not the current weird 3-way choice > > between insn1, insn2 and nothing at all. The only time that makes sense > > is when one of the insns is a branch that skips the block, but that's > > handled via the alternative_if macros instead. > > > > Behaving always like first_insn=1 provides an if-else that is statically > > optimised if the relevant feature is configured out, which I think is > > the only thing people are ever going to want. > > > > Maybe something depends on the current behaviour, but I can't see it so > > far... > > I'll give it a go in v4 and see how it looks. > > Another option would be an alternative_else which takes an enable > argument. Sure, I think it could make sense to have a different wrapper so that the meaning of invocations is clearer for this special case. For the underlying macro, maybe it would be simpler to make it truly 3-way: .macro alternative_insn insn_with_cap:req, insn_without_cap:req, cap:req, \ enable_alternative=1, fallback_insn= // ... .if (\enable_alternative) // as currently .else \fallback_insn .endif .endm Then we can rejig the various frontends around that. If you don't want anything when the alternative is disabled, you just omit fallback_insn. Cheers ---Dave
On Wed, Apr 29, 2020 at 11:26:00AM +0100, Dave P Martin wrote: > On Tue, Apr 28, 2020 at 12:43:54PM +0100, Catalin Marinas wrote: > > On Mon, Apr 27, 2020 at 05:57:37PM +0100, Dave P Martin wrote: > > > On Tue, Apr 21, 2020 at 03:25:41PM +0100, Catalin Marinas wrote: > > > > diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h > > > > index 5e5dc05d63a0..67d7cc608336 100644 > > > > --- a/arch/arm64/include/asm/alternative.h > > > > +++ b/arch/arm64/include/asm/alternative.h > > > > @@ -111,7 +111,11 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > > > > .byte \alt_len > > > > .endm > > > > > > > > -.macro alternative_insn insn1, insn2, cap, enable = 1 > > > > +/* > > > > + * Disable the whole block if enable == 0, unless first_insn == 1 in which > > > > + * case insn1 will always be issued but without an alternative insn2. > > > > + */ > > > > +.macro alternative_insn insn1, insn2, cap, enable = 1, first_insn = 0 > > > > .if \enable > > > > 661: \insn1 > > > > 662: .pushsection .altinstructions, "a" > > > > @@ -122,6 +126,8 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > > > > 664: .popsection > > > > .org . - (664b-663b) + (662b-661b) > > > > .org . - (662b-661b) + (664b-663b) > > > > + .elseif \first_insn > > > > + \insn1 > > > > > > This becomes quite unreadable at the invocation site, especially when > > > invoked as "alternative_insn ..., 1". "... first_insn=1" is not much > > > better either). > > > > That I agree. > > > > The reason I didn't leave the alternative in place here is that if gas > > doesn't support MTE, it will fail to compile. I wanted to avoid the > > several #ifdef's. > > We could solve that by synthesising the opcodes instead of relying on > gas (as we do for other extensions). While in this particular case the instruction takes only one register, we need gas with MTE support anyway for more complex instructions in the other .S files. I don't think it's worth the effort of writing our own assembler in the kernel as macros. > > While this is C code + inline asm, I'd like to have a consistent > > behaviour of ALTERNATIVE between C and .S files. Now, given that some of > > them (like UAO/PAN) are on by default, it probably doesn't make any > > difference if we always keep the first block (non-alternative). > > > > We could add a new macro ALTERNATIVE_OR_NOP. > > alternative_insn doesn't seem exist for C at all. Did I miss something? There is ALTERNATIVE() which is defined for both C and asm (the latter ends up using alternative_insn). > > > Can we instead just always behave as if first_insn=1 instead? This this > > > works intuitively as an alternative, not the current weird 3-way choice > > > between insn1, insn2 and nothing at all. The only time that makes sense > > > is when one of the insns is a branch that skips the block, but that's > > > handled via the alternative_if macros instead. > > > > > > Behaving always like first_insn=1 provides an if-else that is statically > > > optimised if the relevant feature is configured out, which I think is > > > the only thing people are ever going to want. > > > > > > Maybe something depends on the current behaviour, but I can't see it so > > > far... > > > > I'll give it a go in v4 and see how it looks. > > > > Another option would be an alternative_else which takes an enable > > argument. > > Sure, I think it could make sense to have a different wrapper so that > the meaning of invocations is clearer for this special case. > > > For the underlying macro, maybe it would be simpler to make it truly > 3-way: > > .macro alternative_insn insn_with_cap:req, insn_without_cap:req, cap:req, \ > enable_alternative=1, fallback_insn= 'fallback' is an option as well. See below for what it takes to always emit the first instruction in the alternative blocks (replacing this patch). The clear_page() zeroing line would become: ALTERNATIVE("dc zva, x0", "stzgm xzr, [x0]", ARM64_MTE, CONFIG_ARM64_MTE) (or alternative_insn, the above save an IS_ENABLED). --------8<------------------------ From 73f3869cb68fab1505d7b400ae8a39a19c5fc9e9 Mon Sep 17 00:00:00 2001 From: Catalin Marinas <catalin.marinas@arm.com> Date: Wed, 27 Nov 2019 09:07:30 +0000 Subject: [PATCH] arm64: alternative: Always emit the first instruction in ALTERNATIVE blocks Currently with the ALTERNATIVE macro or alternative_insn, the cfg (or enable) arguments disable the entire asm block. Change the macros to only omit the alternative block on !IS_ENABLED(cfg). In addition, remove the cfg arguments to to ALTERNATIVE in those few calls where it is still passed. There is no change to the resulting kernel image with defconfig. alternative_insn's enable argument will be used in a subsequent patch and we are keeping the ALTERNATIVE C macro arguments in line with the asm version. Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> --- arch/arm64/include/asm/alternative.h | 13 ++++++++----- arch/arm64/include/asm/tlbflush.h | 6 ++---- arch/arm64/include/asm/uaccess.h | 15 +++++---------- arch/arm64/kvm/hyp/entry.S | 2 +- 4 files changed, 16 insertions(+), 20 deletions(-) diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h index 5e5dc05d63a0..ecb44cb0d6b1 100644 --- a/arch/arm64/include/asm/alternative.h +++ b/arch/arm64/include/asm/alternative.h @@ -66,9 +66,9 @@ static inline void apply_alternatives_module(void *start, size_t length) { } * Alternatives with callbacks do not generate replacement instructions. */ #define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled) \ - ".if "__stringify(cfg_enabled)" == 1\n" \ "661:\n\t" \ oldinstr "\n" \ + ".if "__stringify(cfg_enabled)" == 1\n" \ "662:\n" \ ".pushsection .altinstructions,\"a\"\n" \ ALTINSTR_ENTRY(feature) \ @@ -83,9 +83,9 @@ static inline void apply_alternatives_module(void *start, size_t length) { } ".endif\n" #define __ALTERNATIVE_CFG_CB(oldinstr, feature, cfg_enabled, cb) \ - ".if "__stringify(cfg_enabled)" == 1\n" \ "661:\n\t" \ oldinstr "\n" \ + ".if "__stringify(cfg_enabled)" == 1\n" \ "662:\n" \ ".pushsection .altinstructions,\"a\"\n" \ ALTINSTR_ENTRY_CB(feature, cb) \ @@ -111,9 +111,12 @@ static inline void apply_alternatives_module(void *start, size_t length) { } .byte \alt_len .endm +/* + * If enable == 0, the alternative block will be omitted. + */ .macro alternative_insn insn1, insn2, cap, enable = 1 - .if \enable 661: \insn1 + .if \enable 662: .pushsection .altinstructions, "a" altinstruction_entry 661b, 663f, \cap, 662b-661b, 664f-663f .popsection @@ -289,8 +292,8 @@ alternative_endif * Usage: asm(ALTERNATIVE(oldinstr, newinstr, feature)); * * Usage: asm(ALTERNATIVE(oldinstr, newinstr, feature, CONFIG_FOO)); - * N.B. If CONFIG_FOO is specified, but not selected, the whole block - * will be omitted, including oldinstr. + * N.B. If CONFIG_FOO is specified, but not selected, the alternative block + * will be omitted. */ #define ALTERNATIVE(oldinstr, newinstr, ...) \ _ALTERNATIVE_CFG(oldinstr, newinstr, __VA_ARGS__, 1) diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h index bc3949064725..8c79f12900ce 100644 --- a/arch/arm64/include/asm/tlbflush.h +++ b/arch/arm64/include/asm/tlbflush.h @@ -30,15 +30,13 @@ #define __TLBI_0(op, arg) asm ("tlbi " #op "\n" \ ALTERNATIVE("nop\n nop", \ "dsb ish\n tlbi " #op, \ - ARM64_WORKAROUND_REPEAT_TLBI, \ - CONFIG_ARM64_WORKAROUND_REPEAT_TLBI) \ + ARM64_WORKAROUND_REPEAT_TLBI) \ : : ) #define __TLBI_1(op, arg) asm ("tlbi " #op ", %0\n" \ ALTERNATIVE("nop\n nop", \ "dsb ish\n tlbi " #op ", %0", \ - ARM64_WORKAROUND_REPEAT_TLBI, \ - CONFIG_ARM64_WORKAROUND_REPEAT_TLBI) \ + ARM64_WORKAROUND_REPEAT_TLBI) \ : : "r" (arg)) #define __TLBI_N(op, arg, n, ...) __TLBI_##n(op, arg) diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h index 32fc8061aa76..d1812cdaab01 100644 --- a/arch/arm64/include/asm/uaccess.h +++ b/arch/arm64/include/asm/uaccess.h @@ -45,8 +45,7 @@ static inline void set_fs(mm_segment_t fs) if (IS_ENABLED(CONFIG_ARM64_UAO) && fs == KERNEL_DS) asm(ALTERNATIVE("nop", SET_PSTATE_UAO(1), ARM64_HAS_UAO)); else - asm(ALTERNATIVE("nop", SET_PSTATE_UAO(0), ARM64_HAS_UAO, - CONFIG_ARM64_UAO)); + asm(ALTERNATIVE("nop", SET_PSTATE_UAO(0), ARM64_HAS_UAO)); } #define segment_eq(a, b) ((a) == (b)) @@ -175,28 +174,24 @@ static inline bool uaccess_ttbr0_enable(void) static inline void __uaccess_disable_hw_pan(void) { - asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN, - CONFIG_ARM64_PAN)); + asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN)); } static inline void __uaccess_enable_hw_pan(void) { - asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN, - CONFIG_ARM64_PAN)); + asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN)); } #define __uaccess_disable(alt) \ do { \ if (!uaccess_ttbr0_disable()) \ - asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), alt, \ - CONFIG_ARM64_PAN)); \ + asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), alt)); \ } while (0) #define __uaccess_enable(alt) \ do { \ if (!uaccess_ttbr0_enable()) \ - asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt, \ - CONFIG_ARM64_PAN)); \ + asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt)); \ } while (0) static inline void uaccess_disable(void) diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S index d22d0534dd60..88b096c18223 100644 --- a/arch/arm64/kvm/hyp/entry.S +++ b/arch/arm64/kvm/hyp/entry.S @@ -109,7 +109,7 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL) add x1, x1, #VCPU_CONTEXT - ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN) + ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN) // Store the guest regs x2 and x3 stp x2, x3, [x1, #CPU_XREG_OFFSET(2)]
On Wed, Apr 29, 2020 at 03:04:25PM +0100, Catalin Marinas wrote: > From 73f3869cb68fab1505d7b400ae8a39a19c5fc9e9 Mon Sep 17 00:00:00 2001 > From: Catalin Marinas <catalin.marinas@arm.com> > Date: Wed, 27 Nov 2019 09:07:30 +0000 > Subject: [PATCH] arm64: alternative: Always emit the first instruction in > ALTERNATIVE blocks > > Currently with the ALTERNATIVE macro or alternative_insn, the cfg (or > enable) arguments disable the entire asm block. Change the macros to > only omit the alternative block on !IS_ENABLED(cfg). In addition, remove > the cfg arguments to to ALTERNATIVE in those few calls where it is still > passed. There is no change to the resulting kernel image with defconfig. > > alternative_insn's enable argument will be used in a subsequent patch > and we are keeping the ALTERNATIVE C macro arguments in line with the > asm version. > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> > Cc: Will Deacon <will@kernel.org> I no longer need this patch as I moved the page tag zeroing from clear_page to the actual mte.S file (called via set_pte_at()). So don't bother reviewing this.
diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h index 5e5dc05d63a0..67d7cc608336 100644 --- a/arch/arm64/include/asm/alternative.h +++ b/arch/arm64/include/asm/alternative.h @@ -111,7 +111,11 @@ static inline void apply_alternatives_module(void *start, size_t length) { } .byte \alt_len .endm -.macro alternative_insn insn1, insn2, cap, enable = 1 +/* + * Disable the whole block if enable == 0, unless first_insn == 1 in which + * case insn1 will always be issued but without an alternative insn2. + */ +.macro alternative_insn insn1, insn2, cap, enable = 1, first_insn = 0 .if \enable 661: \insn1 662: .pushsection .altinstructions, "a" @@ -122,6 +126,8 @@ static inline void apply_alternatives_module(void *start, size_t length) { } 664: .popsection .org . - (664b-663b) + (662b-661b) .org . - (662b-661b) + (664b-663b) + .elseif \first_insn + \insn1 .endif .endm
There are situations where we do not want to disable the whole block based on a config option, only the alternative part while keeping the first instruction. Improve the alternative_insn assembler macro to take a 'first_insn' argument, default 0, to preserve the current behaviour. Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> --- arch/arm64/include/asm/alternative.h | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)