linux-kernel - Re: [PATCH] KVM: arm64: fix u64_replace

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Message-ID: <86h5zj9laj.wl-maz@kernel.org>
Date: Fri, 11 Jul 2025 09:36:04 +0100
From: Marc Zyngier <maz@...nel.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Mark Brown <broonie@...nel.org>,
	James Morse <james.morse@....com>,
	Sebastian Ott <sebott@...hat.com>,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: fix u64_replace_bits() usage

On Fri, 11 Jul 2025 08:27:47 +0100,
Arnd Bergmann <arnd@...nel.org> wrote:
> 
> From: Arnd Bergmann <arnd@...db.de>
> 
> u64_replace_bits() returns a modified word but does not actually modify
> its argument, as pointed out by this new warning:
> 
> arch/arm64/kvm/sys_regs.c: In function 'access_mdcr':
> arch/arm64/kvm/sys_regs.c:2654:17: error: ignoring return value of 'u64_replace_bits' declared with attribute 'warn_unused_result' [-Werror=unused-result]
>  2654 |                 u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
> 
> The intention here must have been to update 'val', so do that instead.
> 
> Fixes: efff9dd2fee7 ("KVM: arm64: Handle out-of-bound write to MDCR_EL2.HPMN")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/arm64/kvm/sys_regs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 33aa4f5071b8..793fb19bebd6 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2651,7 +2651,7 @@ static bool access_mdcr(struct kvm_vcpu *vcpu,
>  	 */
>  	if (hpmn > vcpu->kvm->arch.nr_pmu_counters) {
>  		hpmn = vcpu->kvm->arch.nr_pmu_counters;
> -		u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
> +		val = u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
>  	}
>  
>  	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);

This is only in -next, right? Because I have a fix for this already
queued for 6.16, as per [1].

Thanks,

	M.

[1] https://lore.kernel.org/r/20250709093808.920284-2-ben.horgan@arm.com

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