Re: [PATCH v2 3/3] RISC-V: add T-Head vector errata handling

["Heiko Stübner" <heiko@sntech.de>]

Hey Conor,

Am Freitag, 23. Juni 2023, 11:49:41 CEST schrieb Conor Dooley:
> On Fri, Jun 23, 2023 at 01:13:05AM +0200, Heiko Stuebner wrote:
> > From: Heiko Stuebner <heiko.stuebner@vrull.eu>
> > 
> > T-Head C9xx cores implement an older version (0.7.1) of the vector
> > specification.
> > 
> > Relevant changes concerning the kernel are:
> > - different placement of the SR_VS bit for the vector unit status
> > - different encoding of the vsetvli instruction
> > - different instructions for loads and stores
> > 
> > And a fixed VLEN of 128.
> > 
> > The in-kernel access to vector instances is limited to the save and
> > restore of process states so the above mentioned areas can simply be
> > handled via the alternatives framework, similar to other T-Head specific
> > issues.
> > 
> > Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
> > ---
> >  arch/riscv/Kconfig.errata            |  13 +++
> >  arch/riscv/errata/thead/errata.c     |  32 ++++++
> >  arch/riscv/include/asm/csr.h         |  24 ++++-
> >  arch/riscv/include/asm/errata_list.h |  45 ++++++++-
> >  arch/riscv/include/asm/vector.h      | 139 +++++++++++++++++++++++++--
> >  arch/riscv/kernel/vector.c           |   2 +-
> >  6 files changed, 238 insertions(+), 17 deletions(-)
> > 
> > diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> > index 0c8f4652cd82..b461312dd452 100644
> > --- a/arch/riscv/Kconfig.errata
> > +++ b/arch/riscv/Kconfig.errata
> > @@ -77,4 +77,17 @@ config ERRATA_THEAD_PMU
> >  
> >  	  If you don't know what to do here, say "Y".
> >  
> > +config ERRATA_THEAD_VECTOR
> > +	bool "Apply T-Head Vector errata"
> > +	depends on ERRATA_THEAD && RISCV_ISA_V
> > +	default y
> > +	help
> > +	  The T-Head C9xx cores implement an earlier version 0.7.1
> > +	  of the vector extensions.
> > +
> > +	  This will apply the necessary errata to handle the non-standard
> > +	  behaviour via when switch to and from vector mode for processes.
> > +
> > +	  If you don't know what to do here, say "Y".
> > +
> >  endmenu # "CPU errata selection"
> > diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> > index c259dc925ec1..c41ec84bc8a5 100644
> > --- a/arch/riscv/errata/thead/errata.c
> > +++ b/arch/riscv/errata/thead/errata.c
> > @@ -15,6 +15,7 @@
> >  #include <asm/errata_list.h>
> >  #include <asm/hwprobe.h>
> >  #include <asm/patch.h>
> > +#include <asm/vector.h>
> >  #include <asm/vendorid_list.h>
> >  
> >  static bool errata_probe_pbmt(unsigned int stage,
> > @@ -66,6 +67,34 @@ static bool errata_probe_pmu(unsigned int stage,
> >  	return true;
> >  }
> >  
> > +static bool errata_probe_vector(unsigned int stage,
> > +				unsigned long arch_id, unsigned long impid)
> > +{
> > +	if (!IS_ENABLED(CONFIG_ERRATA_THEAD_VECTOR))
> > +		return false;
> > +
> > +	/* target-c9xx cores report arch_id and impid as 0 */
> > +	if (arch_id != 0 || impid != 0)
> > +		return false;
> > +
> > +	if (stage == RISCV_ALTERNATIVES_EARLY_BOOT) {
> > +		/*
> > +		 * Disable VECTOR to detect illegal usage of vector in kernel.
> > +		 * This is normally done in _start_kernel but with the
> > +		 * vector-1.0 SR_VS bits. VS is using [24:23] on T-Head's
> > +		 * vector-0.7.1 and the vector-1.0-bits are unused there.
> > +		 */
> > +		csr_clear(CSR_STATUS, SR_VS_THEAD);
> > +		return false;
> > +	}
> > +
> > +	/* let has_vector() return true and set the static vlen */
> 
> Hmm, I was wondering about how you were going to communicate this to
> userspace, since you're not going to be setting "v" in your DT, so
> there'll be nothing in /proc/cpuinfo indicating it. (I am assuming that
> this is your intention, as you'd not need to drop the alternative-based
> stuff from has_vector() if it wasn't)

I'm working on the assumption that the t-head vector is way to different
from the official vector, that a userspace will definitly need to handle this
in some way specially and we can't claim to use a "real" vector spec.

So in this first step, my goal is to simply allow userspace programs
compiled to use the t-head vector instructions (i.e. 0.7.1 presumably) to
not hang the kernel and do all the necessary bringup and teardown needed
for executing those vector instructions ;-) .


> I don't think you can do this, as things stand, because of how hwprobe
> operates:
> 
> static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
> 			     const struct cpumask *cpus)
> {
> 	...
> 
> 	if (has_vector())
> 		pair->value |= RISCV_HWPROBE_IMA_V;
> 
> 	...
> }
> 
>   * :c:macro:`RISCV_HWPROBE_IMA_V`: The V extension is supported, as defined by
>     version 1.0 of the RISC-V Vector extension manual.
> 
> You'll need to change hwprobe to use has_vector() &&
> riscv_has_extension_unlikely(v), or similar, as the condition for
> reporting.

ah right, and yes I need to adapt hwprobe as you wrote.


> You'll also need some other way to communicate to userspace
> that T-Head's vector stuff is supported, no?

As I said above, baby-steps - not-dying first ;-) .


> I'm also _really_ unconvinced that turning on extensions that were not
> indicated in the DT or via ACPI is something we should be doing. Have I
> missed something here that'd make that assessment inaccurate?

Hmm, DT (and ACPI) is a (static) hardware-description, not a configuration
space (sermon of DT maintainers for years), so the ISA string in DT will
simply describe _all_ extensions the hardware supports. So there _should_
never be a case of "I want to disable vectors and will remove the letter
from the ISA string".

For T-Head we _know_ from vendor-id and friends that the core supports
this special brand of vectors.

We're also turning on the t-head equivalent of svpbmt and zicbom with
probably the same reasoning.


> FWIW I am currently working on kernel-side support for the new
> extension properties that I have posted dt-binding patches for.
> I'll go post it once Palmer has merged his current set of patches in his
> staging repo into for-next, as I've got a lot of deps on riscv,isa
> parser changes.
> I'm really hoping that it provides an easier way to go off probing for
> vendor specific stuff for DT-based systems, since it will no longer
> require complex probing, just an of_property_match_string() for each
> possible cpu and we could very well provide a vendor hook during that
> process.
> Clearly though, that stuff is not yet merged as it has not even been
> posted yet.

And with the comments I received, T-Head vector also is not ready for
prime-time yet, so we're all good :-)


Heiko


> Current WIP of that is here:
> https://git.kernel.org/pub/scm/linux/kernel/git/conor/linux.git/log/?h=riscv-extensions-strings-on-palmer
> 
> > +	riscv_vector_supported();
> > +	riscv_v_vsize = 128 / 8 * 32;
> > +
> > +	return true;
> > +}
> > +
> >  static u32 thead_errata_probe(unsigned int stage,
> >  			      unsigned long archid, unsigned long impid)
> >  {
> > @@ -80,6 +109,9 @@ static u32 thead_errata_probe(unsigned int stage,
> >  	if (errata_probe_pmu(stage, archid, impid))
> >  		cpu_req_errata |= BIT(ERRATA_THEAD_PMU);
> >  
> > +	if (errata_probe_vector(stage, archid, impid))
> > +		cpu_req_errata |= BIT(ERRATA_THEAD_VECTOR);
> > +
> >  	return cpu_req_errata;
> >  }
> >  
> > diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> > index 2d79bca6ffe8..521b3b939e51 100644
> > --- a/arch/riscv/include/asm/csr.h
> > +++ b/arch/riscv/include/asm/csr.h
> > @@ -24,11 +24,25 @@
> >  #define SR_FS_CLEAN	_AC(0x00004000, UL)
> >  #define SR_FS_DIRTY	_AC(0x00006000, UL)
> >  
> > -#define SR_VS		_AC(0x00000600, UL) /* Vector Status */
> > -#define SR_VS_OFF	_AC(0x00000000, UL)
> > -#define SR_VS_INITIAL	_AC(0x00000200, UL)
> > -#define SR_VS_CLEAN	_AC(0x00000400, UL)
> > -#define SR_VS_DIRTY	_AC(0x00000600, UL)
> > +#define SR_VS_OFF		_AC(0x00000000, UL)
> > +
> > +#define SR_VS_1_0		_AC(0x00000600, UL) /* Vector Status */
> > +#define SR_VS_INITIAL_1_0	_AC(0x00000200, UL)
> > +#define SR_VS_CLEAN_1_0		_AC(0x00000400, UL)
> > +#define SR_VS_DIRTY_1_0		_AC(0x00000600, UL)
> > +
> > +#define SR_VS_THEAD		_AC(0x01800000, UL) /* Vector Status */
> > +#define SR_VS_INITIAL_THEAD	_AC(0x00800000, UL)
> > +#define SR_VS_CLEAN_THEAD	_AC(0x01000000, UL)
> > +#define SR_VS_DIRTY_THEAD	_AC(0x01800000, UL)
> > +
> > +/*
> > + * Always default to vector-1.0 handling in assembly and let the broken
> > + * implementations handle their case separately.
> > + */
> > +#ifdef __ASSEMBLY__
> > +#define SR_VS			SR_VS_1_0
> > +#endif
> >  
> >  #define SR_XS		_AC(0x00018000, UL) /* Extension Status */
> >  #define SR_XS_OFF	_AC(0x00000000, UL)
> > diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> > index fb1a810f3d8c..ab21fadbe9c6 100644
> > --- a/arch/riscv/include/asm/errata_list.h
> > +++ b/arch/riscv/include/asm/errata_list.h
> > @@ -21,7 +21,8 @@
> >  #define	ERRATA_THEAD_PBMT 0
> >  #define	ERRATA_THEAD_CMO 1
> >  #define	ERRATA_THEAD_PMU 2
> > -#define	ERRATA_THEAD_NUMBER 3
> > +#define	ERRATA_THEAD_VECTOR 3
> > +#define	ERRATA_THEAD_NUMBER 4
> >  #endif
> >  
> >  #ifdef __ASSEMBLY__
> > @@ -154,6 +155,48 @@ asm volatile(ALTERNATIVE(						\
> >  	: "=r" (__ovl) :						\
> >  	: "memory")
> >  
> > +#ifdef CONFIG_ERRATA_THEAD_VECTOR
> > +
> > +#define THEAD_C9XX_CSR_VXSAT			0x9
> > +#define THEAD_C9XX_CSR_VXRM			0xa
> > +
> > +/*
> > + * Vector 0.7.1 as used for example on T-Head Xuantie cores, uses an older
> > + * encoding for vsetvli (ta, ma vs. d1), so provide an instruction for
> > + * vsetvli	t4, x0, e8, m8, d1
> > + */
> > +#define THEAD_VSETVLI_T4X0E8M8D1	".long	0x00307ed7\n\t"
> > +
> > +/*
> > + * While in theory, the vector-0.7.1 vsb.v and vlb.v result in the same
> > + * encoding as the standard vse8.v and vle8.v, compilers seem to optimize
> > + * the call resulting in a different encoding and then using a value for
> > + * the "mop" field that is not part of vector-0.7.1
> > + * So encode specific variants for vstate_save and _restore.
> > + */
> > +#define THEAD_VSB_V_V0T0		".long	0x02028027\n\t"
> > +#define THEAD_VSB_V_V8T0		".long	0x02028427\n\t"
> > +#define THEAD_VSB_V_V16T0		".long	0x02028827\n\t"
> > +#define THEAD_VSB_V_V24T0		".long	0x02028c27\n\t"
> > +#define THEAD_VLB_V_V0T0		".long	0x012028007\n\t"
> > +#define THEAD_VLB_V_V8T0		".long	0x012028407\n\t"
> > +#define THEAD_VLB_V_V16T0		".long	0x012028807\n\t"
> > +#define THEAD_VLB_V_V24T0		".long	0x012028c07\n\t"
> > +
> > +#define ALT_SR_VS_VECTOR_1_0_SHIFT	9
> > +#define ALT_SR_VS_THEAD_SHIFT		23
> > +
> > +#define ALT_SR_VS(_val, prot)						\
> > +asm(ALTERNATIVE("li %0, %1\t\nslli %0,%0,%3",				\
> > +		"li %0, %2\t\nslli %0,%0,%4", THEAD_VENDOR_ID,		\
> > +		ERRATA_THEAD_VECTOR, CONFIG_ERRATA_THEAD_VECTOR)	\
> > +		: "=r"(_val)						\
> > +		: "I"(prot##_1_0 >> ALT_SR_VS_VECTOR_1_0_SHIFT),	\
> > +		  "I"(prot##_THEAD >> ALT_SR_VS_THEAD_SHIFT),		\
> > +		  "I"(ALT_SR_VS_VECTOR_1_0_SHIFT),			\
> > +		  "I"(ALT_SR_VS_THEAD_SHIFT))
> > +#endif /* CONFIG_ERRATA_THEAD_VECTOR */
> > +
> >  #endif /* __ASSEMBLY__ */
> >  
> >  #endif
> > diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
> > index 315c96d2b4d0..fa47f60f81e3 100644
> > --- a/arch/riscv/include/asm/vector.h
> > +++ b/arch/riscv/include/asm/vector.h
> > @@ -18,6 +18,55 @@
> >  #include <asm/hwcap.h>
> >  #include <asm/csr.h>
> >  #include <asm/asm.h>
> > +#include <asm/errata_list.h>
> > +
> > +#ifdef CONFIG_ERRATA_THEAD_VECTOR
> > +
> > +static inline unsigned long riscv_sr_vs(void)
> > +{
> > +	u32 val;
> > +
> > +	ALT_SR_VS(val, SR_VS);
> > +	return val;
> > +}
> > +
> > +static inline unsigned long riscv_sr_vs_initial(void)
> > +{
> > +	u32 val;
> > +
> > +	ALT_SR_VS(val, SR_VS_INITIAL);
> > +	return val;
> > +}
> > +
> > +static inline unsigned long riscv_sr_vs_clean(void)
> > +{
> > +	u32 val;
> > +
> > +	ALT_SR_VS(val, SR_VS_CLEAN);
> > +	return val;
> > +}
> > +
> > +static inline unsigned long riscv_sr_vs_dirty(void)
> > +{
> > +	u32 val;
> > +
> > +	ALT_SR_VS(val, SR_VS_DIRTY);
> > +	return val;
> > +}
> > +
> > +#define SR_VS		riscv_sr_vs()
> > +#define SR_VS_INITIAL	riscv_sr_vs_initial()
> > +#define SR_VS_CLEAN	riscv_sr_vs_clean()
> > +#define SR_VS_DIRTY	riscv_sr_vs_dirty()
> > +
> > +#else /* CONFIG_ERRATA_THEAD_VECTOR */
> > +
> > +#define SR_VS		SR_VS_1_0
> > +#define SR_VS_INITIAL	SR_VS_INITIAL_1_0
> > +#define SR_VS_CLEAN	SR_VS_CLEAN_1_0
> > +#define SR_VS_DIRTY	SR_VS_DIRTY_1_0
> > +
> > +#endif /* CONFIG_ERRATA_THEAD_VECTOR */
> >  
> >  extern bool riscv_v_supported;
> >  void riscv_vector_supported(void);
> > @@ -63,26 +112,74 @@ static __always_inline void riscv_v_disable(void)
> >  
> >  static __always_inline void __vstate_csr_save(struct __riscv_v_ext_state *dest)
> >  {
> > -	asm volatile (
> > +	register u32 t1 asm("t1") = (SR_FS);
> > +
> > +	/*
> > +	 * CSR_VCSR is defined as
> > +	 * [2:1] - vxrm[1:0]
> > +	 * [0] - vxsat
> > +	 * The earlier vector spec implemented by T-Head uses separate
> > +	 * registers for the same bit-elements, so just combine those
> > +	 * into the existing output field.
> > +	 *
> > +	 * Additionally T-Head cores need FS to be enabled when accessing
> > +	 * the VXRM and VXSAT CSRs, otherwise ending in illegal instructions.
> > +	 * Though the cores do not implement the VXRM and VXSAT fields in the
> > +	 * FCSR CSR that vector-0.7.1 specifies.
> > +	 */
> > +	asm volatile (ALTERNATIVE(
> >  		"csrr	%0, " __stringify(CSR_VSTART) "\n\t"
> >  		"csrr	%1, " __stringify(CSR_VTYPE) "\n\t"
> >  		"csrr	%2, " __stringify(CSR_VL) "\n\t"
> >  		"csrr	%3, " __stringify(CSR_VCSR) "\n\t"
> > +		__nops(5),
> > +		"csrs	sstatus, t1\n\t"
> > +		"csrr	%0, " __stringify(CSR_VSTART) "\n\t"
> > +		"csrr	%1, " __stringify(CSR_VTYPE) "\n\t"
> > +		"csrr	%2, " __stringify(CSR_VL) "\n\t"
> > +		"csrr	%3, " __stringify(THEAD_C9XX_CSR_VXRM) "\n\t"
> > +		"slliw	%3, %3, " __stringify(VCSR_VXRM_SHIFT) "\n\t"
> > +		"csrr	t4, " __stringify(THEAD_C9XX_CSR_VXSAT) "\n\t"
> > +		"or	%3, %3, t4\n\t"
> > +		"csrc	sstatus, t1\n\t",
> > +		THEAD_VENDOR_ID,
> > +		ERRATA_THEAD_VECTOR, CONFIG_ERRATA_THEAD_VECTOR)
> >  		: "=r" (dest->vstart), "=r" (dest->vtype), "=r" (dest->vl),
> > -		  "=r" (dest->vcsr) : :);
> > +		  "=r" (dest->vcsr) : "r"(t1) : "t4");
> >  }
> >  
> >  static __always_inline void __vstate_csr_restore(struct __riscv_v_ext_state *src)
> >  {
> > -	asm volatile (
> > +	register u32 t1 asm("t1") = (SR_FS);
> > +
> > +	/*
> > +	 * Similar to __vstate_csr_save above, restore values for the
> > +	 * separate VXRM and VXSAT CSRs from the vcsr variable.
> > +	 */
> > +	asm volatile (ALTERNATIVE(
> >  		".option push\n\t"
> >  		".option arch, +v\n\t"
> >  		"vsetvl	 x0, %2, %1\n\t"
> >  		".option pop\n\t"
> >  		"csrw	" __stringify(CSR_VSTART) ", %0\n\t"
> >  		"csrw	" __stringify(CSR_VCSR) ", %3\n\t"
> > +		__nops(6),
> > +		"csrs	sstatus, t1\n\t"
> > +		".option push\n\t"
> > +		".option arch, +v\n\t"
> > +		"vsetvl	 x0, %2, %1\n\t"
> > +		".option pop\n\t"
> > +		"csrw	" __stringify(CSR_VSTART) ", %0\n\t"
> > +		"srliw	t4, %3, " __stringify(VCSR_VXRM_SHIFT) "\n\t"
> > +		"andi	t4, t4, " __stringify(VCSR_VXRM_MASK) "\n\t"
> > +		"csrw	" __stringify(THEAD_C9XX_CSR_VXRM) ", t4\n\t"
> > +		"andi	%3, %3, " __stringify(VCSR_VXSAT_MASK) "\n\t"
> > +		"csrw	" __stringify(THEAD_C9XX_CSR_VXSAT) ", %3\n\t"
> > +		"csrc	sstatus, t1\n\t",
> > +		THEAD_VENDOR_ID,
> > +		ERRATA_THEAD_VECTOR, CONFIG_ERRATA_THEAD_VECTOR)
> >  		: : "r" (src->vstart), "r" (src->vtype), "r" (src->vl),
> > -		    "r" (src->vcsr) :);
> > +		    "r" (src->vcsr), "r"(t1) : "t4");
> >  }
> >  
> >  static inline void __riscv_v_vstate_save(struct __riscv_v_ext_state *save_to,
> > @@ -92,7 +189,8 @@ static inline void __riscv_v_vstate_save(struct __riscv_v_ext_state *save_to,
> >  
> >  	riscv_v_enable();
> >  	__vstate_csr_save(save_to);
> > -	asm volatile (
> > +	asm volatile (ALTERNATIVE(
> > +		"nop\n\t"
> >  		".option push\n\t"
> >  		".option arch, +v\n\t"
> >  		"vsetvli	%0, x0, e8, m8, ta, ma\n\t"
> > @@ -103,8 +201,18 @@ static inline void __riscv_v_vstate_save(struct __riscv_v_ext_state *save_to,
> >  		"vse8.v		v16, (%1)\n\t"
> >  		"add		%1, %1, %0\n\t"
> >  		"vse8.v		v24, (%1)\n\t"
> > -		".option pop\n\t"
> > -		: "=&r" (vl) : "r" (datap) : "memory");
> > +		".option pop\n\t",
> > +		"mv		t0, %1\n\t"
> > +		THEAD_VSETVLI_T4X0E8M8D1
> > +		THEAD_VSB_V_V0T0
> > +		"addi		t0, t0, 128\n\t"
> > +		THEAD_VSB_V_V8T0
> > +		"addi		t0, t0, 128\n\t"
> > +		THEAD_VSB_V_V16T0
> > +		"addi		t0, t0, 128\n\t"
> > +		THEAD_VSB_V_V24T0, THEAD_VENDOR_ID,
> > +		ERRATA_THEAD_VECTOR, CONFIG_ERRATA_THEAD_VECTOR)
> > +		: "=&r" (vl) : "r" (datap) : "t0", "t4", "memory");
> >  	riscv_v_disable();
> >  }
> >  
> > @@ -114,7 +222,8 @@ static inline void __riscv_v_vstate_restore(struct __riscv_v_ext_state *restore_
> >  	unsigned long vl;
> >  
> >  	riscv_v_enable();
> > -	asm volatile (
> > +	asm volatile (ALTERNATIVE(
> > +		"nop\n\t"
> >  		".option push\n\t"
> >  		".option arch, +v\n\t"
> >  		"vsetvli	%0, x0, e8, m8, ta, ma\n\t"
> > @@ -125,8 +234,18 @@ static inline void __riscv_v_vstate_restore(struct __riscv_v_ext_state *restore_
> >  		"vle8.v		v16, (%1)\n\t"
> >  		"add		%1, %1, %0\n\t"
> >  		"vle8.v		v24, (%1)\n\t"
> > -		".option pop\n\t"
> > -		: "=&r" (vl) : "r" (datap) : "memory");
> > +		".option pop\n\t",
> > +		"mv		t0, %1\n\t"
> > +		THEAD_VSETVLI_T4X0E8M8D1
> > +		THEAD_VLB_V_V0T0
> > +		"addi		t0, t0, 128\n\t"
> > +		THEAD_VLB_V_V8T0
> > +		"addi		t0, t0, 128\n\t"
> > +		THEAD_VLB_V_V16T0
> > +		"addi		t0, t0, 128\n\t"
> > +		THEAD_VLB_V_V24T0, THEAD_VENDOR_ID,
> > +		ERRATA_THEAD_VECTOR, CONFIG_ERRATA_THEAD_VECTOR)
> > +		: "=&r" (vl) : "r" (datap) : "t0", "t4");
> >  	__vstate_csr_restore(restore_from);
> >  	riscv_v_disable();
> >  }
> > diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> > index 74178fb71805..51726890a4d0 100644
> > --- a/arch/riscv/kernel/vector.c
> > +++ b/arch/riscv/kernel/vector.c
> > @@ -140,7 +140,7 @@ bool riscv_v_first_use_handler(struct pt_regs *regs)
> >  	u32 insn = (u32)regs->badaddr;
> >  
> >  	/* Do not handle if V is not supported, or disabled */
> > -	if (!(ELF_HWCAP & COMPAT_HWCAP_ISA_V))
> > +	if (!has_vector())
> >  		return false;
> >  
> >  	/* If V has been enabled then it is not the first-use trap */
> 





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