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Date: Wed, 15 Jul 2026 12:52:20 -0700
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Subject: Re: [PATCH v3 06/16] arm_mpam: propagate MSC read errors for
 __ris_msmon_read()
To: Andre Przywara <andre.przywara@arm.com>,
 Lorenzo Pieralisi <lpieralisi@kernel.org>, Hanjun Guo
 <guohanjun@huawei.com>, Sudeep Holla <sudeep.holla@kernel.org>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 "Rafael J . Wysocki" <rafael@kernel.org>, Len Brown <lenb@kernel.org>,
 James Morse <james.morse@arm.com>, Ben Horgan <ben.horgan@arm.com>,
 Reinette Chatre <reinette.chatre@intel.com>, Fenghua Yu <fenghuay@nvidia.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
 Srivathsa L Rao <srivathsa.rao@oss.qualcomm.com>,
 Ganapatrao Kulkarni <ganapatrao.kulkarni@oss.qualcomm.com>,
 Trilok Soni <tsoni@quicinc.com>, Srinivas Ramana <sramana@qti.qualcomm.com>,
 Niyas Sait <niyas.sait@arm.com>, linux-acpi@vger.kernel.org,
 linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20260710144520.917375-1-andre.przywara@arm.com>
 <20260710144520.917375-7-andre.przywara@arm.com>
Content-Language: en-US
From: Lee Trager <lee@trager.us>
In-Reply-To: <20260710144520.917375-7-andre.przywara@arm.com>
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On 7/10/26 7:45 AM, Andre Przywara wrote:

> @@ -1748,6 +1758,7 @@ static int mpam_restore_mbwu_state(void *_ris)
>   {
>   	int i;
>   	u64 val;
> +	int ret = 0;
>   	struct mon_read mwbu_arg;
>   	struct mpam_msc_ris *ris = _ris;
>   	struct mpam_class *class = ris->vmsc->comp->class;
> @@ -1760,10 +1771,14 @@ static int mpam_restore_mbwu_state(void *_ris)
>   			mwbu_arg.val = &val;
>   
>   			__ris_msmon_read(&mwbu_arg);
> +			if (mwbu_arg.err) {
> +				ret = mwbu_arg.err;
> +				break;
> +			}
>   		}
>   	}
>   
> -	return 0;
> +	return ret;
>   }
>   
>   /* Call with MSC cfg_lock held */


mwbu_arg is never initialized and __ris_msmon_read() only writes ->err 
on failure. If on a successful read mwb_arg.err tests stack garbage it 
can spuriously break ot of the loop, leaving the remaining monitors 
unrestored, and return a junk error code.

Setting `struct mon_read mwbu_arg = {};` matches what mpam_msmon_read() 
already does. Additionally val should be initialized to 0 too. 
__ris_msmon_reaD() does *m->val += now, and although the result is 
discarded here, KMSAN would flag the uninitialized read.