Re: [PATCH bpf-next 2/5] riscv, bpf: Relax restrictions on Zbb instructions

Conor Dooley
2024-03-29T11:24:02Z

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On Thu, Mar 28, 2024 at 10:07:23PM +0000, Conor Dooley wrote:

> As I said on IRC to you earlier, I think the Kconfig options here are in
> need of a bit of a spring cleaning - they should be modified to explain
> their individual purposes, be that enabling optimisations in the kernel
> or being required for userspace. I'll try to send a patch for that if
> I remember tomorrow.

Something like this:

-- >8 --
commit 5125504beaedd669b082bf74b02003a77360670f
Author: Conor Dooley <conor.dooley@microchip.com>
Date: Fri Mar 29 11:13:22 2024 +0000

RISC-V: clarify what some RISCV_ISA* config options do

During some discussion on IRC yesterday and on Pu's bpf patch [1]
I noticed that these RISCV_ISA* Kconfig options are not really clear
about their implications. Many of these options have no impact on what
userspace is allowed to do, for example an application can use Zbb
regardless of whether or not the kernel does. Change the help text to
try and clarify whether or not an option affects just the kernel, or
also userspace. None of these options actually control whether or not an
extension is detected dynamically as that's done regardless of Kconfig
options, so drop any text that implies the option is required for
dynamic detection, rewording them as "do x when y is detected".

Link: https://lore.kernel.org/linux-riscv/20240328-ferocity-repose-c554f75a676c@spud/ [1]
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
---
I did this based on top of Samuel's changes dropping the MMU
requurements just in case, but I don't think there's a conflict:
https://lore.kernel.org/linux-riscv/20240227003630.3634533-4-samuel.holland@sifive.com/

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d8a777f59402..f327a8ac648f 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -501,8 +501,8 @@ config RISCV_ISA_SVNAPOT
depends on RISCV_ALTERNATIVE
default y
help
- Allow kernel to detect the Svnapot ISA-extension dynamically at boot
- time and enable its usage.
+ Add support for the Svnapot ISA-extension when it is detected by
+ the kernel at boot.

The Svnapot extension is used to mark contiguous PTEs as a range
of contiguous virtual-to-physical translations for a naturally
@@ -520,9 +520,9 @@ config RISCV_ISA_SVPBMT
depends on RISCV_ALTERNATIVE
default y
help
- Adds support to dynamically detect the presence of the Svpbmt
- ISA-extension (Supervisor-mode: page-based memory types) and
- enable its usage.
+ Add support for the Svpbmt ISA-extension (Supervisor-mode:
+ page-based memory types) when it is detected by the kernel at
+ boot.

The memory type for a page contains a combination of attributes
that indicate the cacheability, idempotency, and ordering
@@ -541,14 +541,15 @@ config TOOLCHAIN_HAS_V
depends on AS_HAS_OPTION_ARCH

config RISCV_ISA_V
- bool "VECTOR extension support"
+ bool "Vector extension support"
depends on TOOLCHAIN_HAS_V
depends on FPU
select DYNAMIC_SIGFRAME
default y
help
Say N here if you want to disable all vector related procedure
- in the kernel.
+ in the kernel. Without this option enabled, neither the kernel nor
+ userspace may use vector.

If you don't know what to do here, say Y.

@@ -606,8 +607,8 @@ config RISCV_ISA_ZBB
depends on RISCV_ALTERNATIVE
default y
help
- Adds support to dynamically detect the presence of the ZBB
- extension (basic bit manipulation) and enable its usage.
+ Add support for enabling optimisations in the kernel when the
+ Zbb extension is detected at boot.

The Zbb extension provides instructions to accelerate a number
of bit-specific operations (count bit population, sign extending,
@@ -623,9 +624,9 @@ config RISCV_ISA_ZICBOM
select RISCV_DMA_NONCOHERENT
select DMA_DIRECT_REMAP
help
- Adds support to dynamically detect the presence of the ZICBOM
- extension (Cache Block Management Operations) and enable its
- usage.
+ Add support for the Zicbom extension (Cache Block Management
+ Operations) and enable its use in the kernel when it is detected
+ at boot.

The Zicbom extension can be used to handle for example
non-coherent DMA support on devices that need it.
@@ -684,7 +685,8 @@ config FPU
default y
help
Say N here if you want to disable all floating-point related procedure
- in the kernel.
+ in the kernel. Without this option enabled, neither the kernel nor
+ userspace may use vector.

If you don't know what to do here, say Y.

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