linux-kernel - Re: [PATCH v2 3/8] riscv: Add PLATFORM_SUPPORTS_RISCV_ISA

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Date: Wed, 8 May 2024 10:00:48 +0100
From: Ben Dooks <ben.dooks@...ethink.co.uk>
To: Charlie Jenkins <charlie@...osinc.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Conor Dooley <conor.dooley@...rochip.com>, Song Liu <song@...nel.org>,
 Xi Wang <xi.wang@...il.com>, Björn Töpel
 <bjorn@...osinc.com>, Clément Léger
 <cleger@...osinc.com>, Jessica Clarke <jrtc27@...c27.com>,
 Andy Chiu <andy.chiu@...ive.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/8] riscv: Add PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT
 Kconfig option

On 08/05/2024 02:36, Charlie Jenkins wrote:
> The existing "RISCV_ISA_SVNAPOT" option is repurposed to be used to by
> kernel code to determine if either
> PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT or
> PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT has been set.
> 
> PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT will check if the hardware
> supports Svnapot before using it, while
> PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT will assume that the hardware
> supports Svnapot.
> 
> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> ---
>   arch/riscv/Kconfig               | 19 -----------------
>   arch/riscv/Kconfig.isa           | 44 ++++++++++++++++++++++++++++++++++++++++
>   arch/riscv/include/asm/pgtable.h |  3 ++-
>   3 files changed, 46 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index c2e9eded0a7d..3c1960e8cd7c 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -484,25 +484,6 @@ config RISCV_ALTERNATIVE_EARLY
>   	help
>   	  Allows early patching of the kernel for special errata
>   
> -config RISCV_ISA_SVNAPOT
> -	bool "Svnapot extension support for supervisor mode NAPOT pages"
> -	depends on 64BIT && MMU
> -	depends on RISCV_ALTERNATIVE
> -	default y
> -	help
> -	  Add support for the Svnapot ISA-extension in the kernel when it
> -	  is detected at boot.
> -
> -	  The Svnapot extension is used to mark contiguous PTEs as a range
> -	  of contiguous virtual-to-physical translations for a naturally
> -	  aligned power-of-2 (NAPOT) granularity larger than the base 4KB page
> -	  size. When HUGETLBFS is also selected this option unconditionally
> -	  allocates some memory for each NAPOT page size supported by the kernel.
> -	  When optimizing for low memory consumption and for platforms without
> -	  the Svnapot extension, it may be better to say N here.
> -
> -	  If you don't know what to do here, say Y.
> -
>   config RISCV_ISA_SVPBMT
>   	bool "Svpbmt extension support for supervisor mode page-based memory types"
>   	depends on 64BIT && MMU
> diff --git a/arch/riscv/Kconfig.isa b/arch/riscv/Kconfig.isa
> index 0663c98b5b17..37585bcd763e 100644
> --- a/arch/riscv/Kconfig.isa
> +++ b/arch/riscv/Kconfig.isa
> @@ -124,3 +124,47 @@ config RISCV_ISA_V_PREEMPTIVE
>   	  This config allows kernel to run SIMD without explicitly disable
>   	  preemption. Enabling this config will result in higher memory
>   	  consumption due to the allocation of per-task's kernel Vector context.
> +
> +config RISCV_ISA_SVNAPOT
> +	bool
> +
> +choice
> +	prompt "Svnapot extension support for supervisor mode NAPOT pages"
> +	default PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT
> +	help
> +	  This selects the level of support for Svnapot in the Linux Kernel.
> +
> +	  The Svnapot extension is used to mark contiguous PTEs as a range
> +	  of contiguous virtual-to-physical translations for a naturally
> +	  aligned power-of-2 (NAPOT) granularity larger than the base 4KB page
> +	  size. When HUGETLBFS is also selected this option unconditionally
> +	  allocates some memory for each NAPOT page size supported by the kernel.
> +	  When optimizing for low memory consumption and for platforms without
> +	  the Svnapot extension, it may be better to prohibit Svnapot.
> +
> +config PROHIBIT_RISCV_ISA_SVNAPOT
> +	bool "Prohibit Svnapot extension"
> +	help
> +	  Regardless of if the platform supports Svnapot, prohibit the kernel
> +	  from using Svnapot.
> +
> +config PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT
> +	bool "Allow Svnapot extension if supported"
> +	depends on 64BIT && MMU
> +	depends on RISCV_ALTERNATIVE
> +	select RISCV_ISA_SVNAPOT
> +	help
> +	  Add support for the Svnapot ISA-extension in the kernel when it
> +	  is detected at boot.
> +
> +config PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT
> +	bool "Emit Svnapot mappings when building Linux"
> +	depends on 64BIT && MMU
> +	depends on NONPORTABLE
> +	select RISCV_ISA_SVNAPOT
> +	help
> +	  Compile a kernel that assumes that the platform supports Svnapot.
> +	  This option produces a kernel that will not run on systems that do
> +	  not support Svnapot.
> +
> +endchoice
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 6afd6bb4882e..432be9691b78 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -289,7 +289,8 @@ static inline pte_t pud_pte(pud_t pud)
>   
>   static __always_inline bool has_svnapot(void)
>   {
> -	return riscv_has_extension_likely(RISCV_ISA_EXT_SVNAPOT);
> +	return IS_ENABLED(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT) ||
> +	       riscv_has_extension_likely(RISCV_ISA_EXT_SVNAPOT);

could you add the IS_ENABLED(*) check into riscv_has_extension_likely
and other such functions?


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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