linux-kernel - Re: [PATCH] arm64: fix cleared E0POE bit after cpu

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Message-ID: <af65f271-5af9-432d-bec9-e44638db9b9f@arm.com>
Date: Wed, 7 Jan 2026 10:43:24 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Yeoreum Yun <yeoreum.yun@....com>, linux-pm@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc: rafael@...nel.org, pavel@...nel.org, catalin.marinas@....com,
 will@...nel.org, anshuman.khandual@....com, ryan.roberts@....com,
 yang@...amperecomputing.com, joey.gouly@....com
Subject: Re: [PATCH] arm64: fix cleared E0POE bit after cpu_suspend()/resume()

On 05/01/2026 21:07, Yeoreum Yun wrote:
> TCR2_ELx.E0POE is set during smp_init().
> However, this bit is not reprogrammed when the CPU enters suspension and
> later resumes via cpu_resume(), as __cpu_setup() does not re-enable E0POE
> and there is no save/restore logic for the TCR2_ELx system register.
>
> As a result, the E0POE feature no longer works after cpu_resume().
>
> To address this, save and restore TCR2_EL1 in the cpu_suspend()/cpu_resume()
> path, rather than adding related logic to __cpu_setup(), taking into account
> possible future extensions of the TCR2_ELx feature.

This is a very good catch!

> Fixes: bf83dae90fbc ("arm64: enable the Permission Overlay Extension for EL0")

We should also Cc: stable@...r.kernel.org as this should be backported
to stable kernels that support POE.

> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
> NOTE:
>   This patch based on v6.19-rc4
> ---
>  arch/arm64/include/asm/suspend.h |  2 +-
>  arch/arm64/mm/proc.S             | 22 ++++++++++++++--------
>  2 files changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/include/asm/suspend.h b/arch/arm64/include/asm/suspend.h
> index e65f33edf9d6..e9ce68d50ba4 100644
> --- a/arch/arm64/include/asm/suspend.h
> +++ b/arch/arm64/include/asm/suspend.h
> @@ -2,7 +2,7 @@
>  #ifndef __ASM_SUSPEND_H
>  #define __ASM_SUSPEND_H
>
> -#define NR_CTX_REGS 13
> +#define NR_CTX_REGS 14
>  #define NR_CALLEE_SAVED_REGS 12
>
>  /*
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index 01e868116448..3888f2ca43fb 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -97,8 +97,11 @@ SYM_FUNC_START(cpu_do_suspend)
>  	mrs	x9, mdscr_el1
>  	mrs	x10, oslsr_el1
>  	mrs	x11, sctlr_el1
> -	get_this_cpu_offset x12
> -	mrs	x13, sp_el0
> +alternative_if ARM64_HAS_TCR2
> +	mrs	x12, REG_TCR2_EL1
> +alternative_else_nop_endif
> +	get_this_cpu_offset x13
> +	mrs	x14, sp_el0
>  	stp	x2, x3, [x0]
>  	stp	x4, x5, [x0, #16]
>  	stp	x6, x7, [x0, #32]
> @@ -109,7 +112,7 @@ SYM_FUNC_START(cpu_do_suspend)
>  	 * Save x18 as it may be used as a platform register, e.g. by shadow
>  	 * call stack.
>  	 */
> -	str	x18, [x0, #96]
> +	stp	x14, x18, [x0, #96]

If TCR2_EL1 isn't supported, we store and reload an unused arbitrary
value. I think it'd be better to make it all conditional and add it at
the end, something like:

    alternative_if ARM64_HAS_TCR2
        mrs    x2, REG_TCR2_EL1
        str    x2, [x0, #104]
    alternative_else_nop_endif

Same idea on the resume path. This also avoids the noise of renaming
existing registers.

- Kevin

>  	ret
>  SYM_FUNC_END(cpu_do_suspend)
>
> @@ -130,8 +133,8 @@ SYM_FUNC_START(cpu_do_resume)
>  	 * the buffer to minimize the risk of exposure when used for shadow
>  	 * call stack.
>  	 */
> -	ldr	x18, [x0, #96]
> -	str	xzr, [x0, #96]
> +	ldp	x15, x18, [x0, #96]
> +	str	xzr, [x0, #104]
>  	msr	tpidr_el0, x2
>  	msr	tpidrro_el0, x3
>  	msr	contextidr_el1, x4
> @@ -144,10 +147,13 @@ SYM_FUNC_START(cpu_do_resume)
>  	msr	tcr_el1, x8
>  	msr	vbar_el1, x9
>  	msr	mdscr_el1, x10
> +alternative_if ARM64_HAS_TCR2
> +	msr	REG_TCR2_EL1, x13
> +alternative_else_nop_endif
>
>  	msr	sctlr_el1, x12
> -	set_this_cpu_offset x13
> -	msr	sp_el0, x14
> +	set_this_cpu_offset x14
> +	msr	sp_el0, x15
>  	/*
>  	 * Restore oslsr_el1 by writing oslar_el1
>  	 */
> @@ -161,7 +167,7 @@ alternative_if ARM64_HAS_RAS_EXTN
>  	msr_s	SYS_DISR_EL1, xzr
>  alternative_else_nop_endif
>
> -	ptrauth_keys_install_kernel_nosync x14, x1, x2, x3
> +	ptrauth_keys_install_kernel_nosync x15, x1, x2, x3
>  	isb
>  	ret
>  SYM_FUNC_END(cpu_do_resume)
> --
> LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}
>