linux-kernel - Re: [PATCH] riscv: Fix SMP when shadow call stacks are enabled

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Message-ID: <20231123-vision-nuttiness-b75be703f162@wendy>
Date:   Thu, 23 Nov 2023 14:06:55 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Samuel Holland <samuel.holland@...ive.com>
CC:     Palmer Dabbelt <palmer@...belt.com>,
        <linux-riscv@...ts.infradead.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andy Chiu <andy.chiu@...ive.com>,
        Clément Léger <cleger@...osinc.com>,
        Greentime Hu <greentime.hu@...ive.com>,
        Guo Ren <guoren@...nel.org>, Heiko Stuebner <heiko@...ech.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nam Cao <namcaov@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv: Fix SMP when shadow call stacks are enabled

On Tue, Nov 21, 2023 at 01:19:29PM -0800, Samuel Holland wrote:
> This fixes two bugs in SCS initialization for secondary CPUs. First,
> the SCS was not initialized at all in the spinwait boot path. Second,
> the code for the SBI HSM path attempted to initialize the SCS before
> enabling the MMU. However, that involves dereferencing the thread
> pointer, which requires the MMU to be enabled.
> 
> Fix both issues by setting up the SCS in the common secondary entry
> path, after enabling the MMU.

I'm curious, mostly because I do not know much about the implemtnation
of the shadow call stack, but does it actually work correctly when the
kernel is built without mmu support?

> 
> Fixes: d1584d791a29 ("riscv: Implement Shadow Call Stack")
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
> 
>  arch/riscv/kernel/head.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index b77397432403..76ace1e0b46f 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -154,7 +154,6 @@ secondary_start_sbi:
>  	XIP_FIXUP_OFFSET a3
>  	add a3, a3, a1
>  	REG_L sp, (a3)
> -	scs_load_current
>  
>  .Lsecondary_start_common:
>  
> @@ -165,6 +164,7 @@ secondary_start_sbi:
>  	call relocate_enable_mmu
>  #endif
>  	call .Lsetup_trap_vector
> +	scs_load_current
>  	tail smp_callin
>  #endif /* CONFIG_SMP */
>  
> -- 
> 2.42.0
> 

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