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

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Message-ID: <20251217002352.GJ6079@nvidia.com>
Date: Tue, 16 Dec 2025 20:23:52 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: Nicolin Chen <nicolinc@...dia.com>, 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, xueshuai@...ux.alibaba.com
Subject: Re: [PATCH rc v3 1/4] iommu/arm-smmu-v3: Add ignored bits to fix STE
 update sequence

On Tue, Dec 16, 2025 at 10:58:33PM +0000, Mostafa Saleh wrote:
>  	for (i = 0; i != NUM_ENTRY_QWORDS; i++) {
> -		/*
> -		 * Ignored is only used for bits that are used by both entries,
> -		 * otherwise it is sequenced according to the unused entry.
> -		 */
> -		ignored[i] &= target_used[i] & cur_used[i];
> -

It is not functionally the same thing without this..

> And we only clear the bits from cur_used, there is no need to for the
> other mask ignored (ignored[i] &= target_used[i] & cur_used[i])
> - If an ignored bit is not in cur_used it will not impact
>   "cur_used[i] &= ~ignored[i]" as it must be already zero
> - If an ignored bit is not in target_used, it doesn't really matter,
>   we can ignore it anyway, as it is safe to do so.

That was an earlier version, it was switched away to this so as to be
less of a change though the reasoning is sound.

Jason