Re: [PATCH -fixes 2/2] riscv: Save/restore envcfg CSR during CPU suspend

[Stefan O'Rear]

On Sun, Feb 11, 2024, at 9:26 PM, Samuel Holland wrote:
> The value of the [ms]envcfg CSR is lost when entering a nonretentive
> idle state, so the CSR must be rewritten when resuming the CPU.
>
> Because the [ms]envcfg CSR is part of the base RISC-V privileged ISA
> specification, it cannot be detected from the ISA string. However, most
> existing hardware is too old to implement this CSR. As a result, it must
> be probed at runtime.
>
> Extend the logic for the Zicsr ISA extension to probe for the presence
> of specific CSRs. Since the CSR number is encoded as an immediate value
> within the csrr instruction, a switch case is necessary for any CSR that
> must be probed this way. Use the exception table to handle the illegal
> instruction exception raised when the CSR is not implemented.

We support non-conforming extensions, so we can't assume that if an
implementation does not provide the Ss1p12 extension which defines senvcfg,
the corresponding CSR number will not be used for other purposes.

-s

> Cc: stable@xxxxxxxxxx
> Fixes: 43c16d51a19b ("RISC-V: Enable cbo.zero in usermode")
> Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
> ---
>
>  arch/riscv/include/asm/csr.h     | 23 +++++++++++++++++++++++
>  arch/riscv/include/asm/suspend.h |  1 +
>  arch/riscv/kernel/cpufeature.c   | 23 +++++++++++++++++++++++
>  arch/riscv/kernel/suspend.c      |  2 ++
>  4 files changed, 49 insertions(+)
>
> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> index 2468c55933cd..daff95feb817 100644
> --- a/arch/riscv/include/asm/csr.h
> +++ b/arch/riscv/include/asm/csr.h
> @@ -542,6 +542,29 @@
>  			      : "memory");			\
>  })
> 
> +#define ALT_CSR_READ(csr)					\
> +({								\
> +	unsigned long __v;					\
> +	__asm__ __volatile__ (					\
> +		ALTERNATIVE("li %[v], 0", "csrr %[v], %[r]", 0,	\
> +			    csr << 16 | RISCV_ISA_EXT_ZICSR, 1)	\
> +		: [v] "=r" (__v)				\
> +		: [r] "i" (csr)					\
> +		: "memory");					\
> +	__v;							\
> +})
> +
> +#define ALT_CSR_WRITE(csr, val)					\
> +({								\
> +	unsigned long __v = (unsigned long)(val);		\
> +	__asm__ __volatile__ (					\
> +		ALTERNATIVE("nop", "csrw %[r], %[v]", 0,	\
> +			    csr << 16 | RISCV_ISA_EXT_ZICSR, 1)	\
> +		: : [r] "i" (csr), [v] "rK" (__v)		\
> +		: "memory");					\
> +	__v;							\
> +})
> +
>  #endif /* __ASSEMBLY__ */
> 
>  #endif /* _ASM_RISCV_CSR_H */
> diff --git a/arch/riscv/include/asm/suspend.h b/arch/riscv/include/asm/suspend.h
> index 02f87867389a..491296a335d0 100644
> --- a/arch/riscv/include/asm/suspend.h
> +++ b/arch/riscv/include/asm/suspend.h
> @@ -14,6 +14,7 @@ struct suspend_context {
>  	struct pt_regs regs;
>  	/* Saved and restored by high-level functions */
>  	unsigned long scratch;
> +	unsigned long envcfg;
>  	unsigned long tvec;
>  	unsigned long ie;
>  #ifdef CONFIG_MMU
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index c5b13f7dd482..934090270ae5 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -954,6 +954,27 @@ void riscv_user_isa_enable(void)
>  }
> 
>  #ifdef CONFIG_RISCV_ALTERNATIVE
> +static bool riscv_cpufeature_probe_csr(u16 csr)
> +{
> +	bool ret = false;
> +
> +	switch (csr) {
> +#define PROBE_CSR_CASE(_csr)			\
> +	case _csr:				\
> +		asm("1:	csrr zero, %[csr]\n"	\
> +		    "	li %[r], 1\n"		\
> +		    "2:\n"			\
> +		    _ASM_EXTABLE(1b, 2b)	\
> +			: [r] "+r" (ret)	\
> +			: [csr] "i" (_csr));	\
> +		break
> +	PROBE_CSR_CASE(CSR_ENVCFG);
> +#undef PROBE_CSR_CASE
> +	}
> +
> +	return ret;
> +}
> +
>  /*
>   * Alternative patch sites consider 48 bits when determining when to patch
>   * the old instruction sequence with the new. These bits are broken into a
> @@ -974,6 +995,8 @@ static bool riscv_cpufeature_patch_check(u16 id, u16 value)
>  		return true;
> 
>  	switch (id) {
> +	case RISCV_ISA_EXT_ZICSR:
> +		return riscv_cpufeature_probe_csr(value);
>  	case RISCV_ISA_EXT_ZICBOZ:
>  		/*
>  		 * Zicboz alternative applications provide the maximum
> diff --git a/arch/riscv/kernel/suspend.c b/arch/riscv/kernel/suspend.c
> index 239509367e42..fe544f12a5c5 100644
> --- a/arch/riscv/kernel/suspend.c
> +++ b/arch/riscv/kernel/suspend.c
> @@ -15,6 +15,7 @@
>  void suspend_save_csrs(struct suspend_context *context)
>  {
>  	context->scratch = csr_read(CSR_SCRATCH);
> +	context->envcfg = ALT_CSR_READ(CSR_ENVCFG);
>  	context->tvec = csr_read(CSR_TVEC);
>  	context->ie = csr_read(CSR_IE);
> 
> @@ -36,6 +37,7 @@ void suspend_save_csrs(struct suspend_context *context)
>  void suspend_restore_csrs(struct suspend_context *context)
>  {
>  	csr_write(CSR_SCRATCH, context->scratch);
> +	ALT_CSR_WRITE(CSR_ENVCFG, context->envcfg);
>  	csr_write(CSR_TVEC, context->tvec);
>  	csr_write(CSR_IE, context->ie);
> 
> -- 
> 2.43.0
>
>
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