Re: [PATCH] KVM: arm64: fix u64_replace_bits() usage

[Marc Zyngier <maz@kernel.org>]

On Fri, 11 Jul 2025 08:27:47 +0100,
Arnd Bergmann <arnd@kernel.org> wrote:
> 
> From: Arnd Bergmann <arnd@arndb.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@arndb.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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