linux-kernel - Re: [PATCH rc v1 1/4] iommu/arm-smmu-v3: Add ignored bits to fix STE update sequence

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Message-ID: <20251206193408.GD1219718@nvidia.com>
Date: Sat, 6 Dec 2025 15:34:08 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: will@...nel.org, robin.murphy@....com, joro@...tes.org,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, skolothumtho@...dia.com,
	praan@...gle.com
Subject: Re: [PATCH rc v1 1/4] iommu/arm-smmu-v3: Add ignored bits to fix STE
 update sequence

On Fri, Dec 05, 2025 at 04:52:00PM -0800, Nicolin Chen wrote:
> @@ -1106,16 +1115,17 @@ static u8 arm_smmu_entry_qword_diff(struct arm_smmu_entry_writer *writer,
>  		 * allowed to set a bit to 1 if the used function doesn't say it
>  		 * is used.
>  		 */
> -		WARN_ON_ONCE(target[i] & ~target_used[i]);
> +		WARN_ON_ONCE(target[i] & ~target_used[i] & ~ignored[i]);
>  
>  		/* Bits can change because they are not currently being used */
> -		unused_update[i] = (entry[i] & cur_used[i]) |
> +		unused_update[i] = (entry[i] & (cur_used[i] | ignored[i])) |
>  				   (target[i] & ~cur_used[i]);

This can't be right? We don't want to ever copy an ignored bit from
entry, the ignored bits should always come from target. The line
should be left alone.

Jason