Message-ID: <4f79f56a-4019-46eb-91dc-f137b714acb7@arm.com>
Date: Tue, 20 Aug 2024 13:43:12 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Jinjie Ruan <ruanjinjie@...wei.com>
Cc: catalin.marinas@....com, will@...nel.org, oleg@...hat.com,
 tglx@...utronix.de, peterz@...radead.org, luto@...nel.org, kees@...nel.org,
 wad@...omium.org, rostedt@...dmis.org, arnd@...db.de, ardb@...nel.org,
 broonie@...nel.org, mark.rutland@....com, rick.p.edgecombe@...el.com,
 leobras@...hat.com, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 3/3] arm64: entry: Convert to generic entry

On 29/06/2024 10:56, Jinjie Ruan wrote:
> Currently, x86, Riscv, Loongarch use the generic entry. Convert arm64
> to use the generic entry infrastructure from kernel/entry/*. The generic
> entry makes maintainers' work easier and codes more elegant, which also
> removed duplicate 150+ LOC. The changes are below:
>
>  - Remove TIF_SYSCALL_* flag, _TIF_WORK_MASK, _TIF_SYSCALL_WORK
>
>  - Remove syscall_trace_enter/exit() and use generic one.
>
>  - Remove *enter_from/exit_to_kernel_mode(), and wrap with generic
>    irqentry_enter/exit().
>
>  - Remove *enter_from/exit_to_user_mode(), and wrap with generic
>    irqentry_enter_from/exit_to_user_mode().
>
>  - Remove arm64_enter/exit_nmi() and use generic irqentry_nmi_enter/exit().
>
>  - Remove PREEMPT_DYNAMIC code, as generic entry will do it ok by
>    implementing arch_irqentry_exit_need_resched().

This is looking good to me overall, moving to using the generic helpers
is a clear improvement. I've tried my best to check that the new
implementation is functionally equivalent to the old. Ignoring additions
in the generic code (such as additional instrumentation_{begin,end}()
pairs or assertions), this seems to be the case, with one exception. The
MTE operations are done in a slightly different order on entry:

* mte_disable_tco_entry() was called *after* the generic lockdep/CT
functions in __enter_from_user_mode(). It is now called before those
generic operations - arch_enter_from_user_mode() called at the beginning
of enter_from_user_mode().

* Similarly mte_disable_tco_entry() and mte_check_tfsr_entry() was
called after the generic operations in enter_from_kernel_mode(), and
they are now called after - arch_enter_from_kernel_mode() called at the
beginning of irqentry_enter().

I am not under the impression that these ordering changes are
problematic, but I may be missing something.

> [...]
>  
> -/*
> - * Handle IRQ/context state management when entering from kernel mode.
> - * Before this function is called it is not safe to call regular kernel code,
> - * instrumentable code, or any code which may trigger an exception.
> - *
> - * This is intended to match the logic in irqentry_enter(), handling the kernel
> - * mode transitions only.
> - */
> -static __always_inline void __enter_from_kernel_mode(struct pt_regs *regs)
> -{
> -	regs->exit_rcu = false;

exit_rcu in struct pt_regs is unused now that these functions are gone
so it can be removed.

> [...]
>  
> @@ -259,48 +74,6 @@ static void noinstr arm64_exit_el1_dbg(struct pt_regs *regs)

arm64_{enter,exit}_el1_dbg() have apparently no generic counterparts we
can replace them with, but maybe we could align them with the generic
functions some more? Specifically, I'm thinking about making them
return/take an irqentry_state_t just like irqentry_nmi_{enter,exit}().
This way we can get rid of struct pt_regs::lockdep_hardirqs, which is
now only used by those functions.

>  		lockdep_hardirqs_on(CALLER_ADDR0);
>  }
>  
> -#ifdef CONFIG_PREEMPT_DYNAMIC
> -DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);

The key is declared in asm/preempt.h, that declaration should also be
removed.

Kevin

> [...]

Powered by blists - more mailing lists