linux-kernel - Re: [PATCH 3/3] iommu/vt-d: Rework hitless PASID entry replacement

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Message-ID: <20260113150542.GF812923@nvidia.com>
Date: Tue, 13 Jan 2026 11:05:42 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>,
	Dmytro Maluka <dmaluka@...omium.org>,
	Samiullah Khawaja <skhawaja@...gle.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] iommu/vt-d: Rework hitless PASID entry replacement

On Tue, Jan 13, 2026 at 11:00:48AM +0800, Lu Baolu wrote:
> +static inline bool pasid_support_hitless_replace(struct pasid_entry *pte,
> +						 struct pasid_entry *new, int type)
> +{
> +	switch (type) {
> +	case PASID_ENTRY_PGTT_FL_ONLY:
> +	case PASID_ENTRY_PGTT_NESTED:
> +		/* The first 128 bits remain the same. */
> +		return READ_ONCE(pte->val[0]) == READ_ONCE(new->val[0]) &&
> +			READ_ONCE(pte->val[1]) == READ_ONCE(new->val[1]);

pte->val128[0] == new->val128[0]

> +	case PASID_ENTRY_PGTT_SL_ONLY:
> +	case PASID_ENTRY_PGTT_PT:
> +		/* The second 128 bits remain the same. */
> +		return READ_ONCE(pte->val[2]) == READ_ONCE(new->val[2]) &&
> +			READ_ONCE(pte->val[3]) == READ_ONCE(new->val[3]);

These READ_ONCE's are pointless, especially the ones on new.

With 5 words to worry about I really feel strongly this should just
use the ARM algorithm. It handles everything very elegantly, we can
lift it out of ARM and make it general.

Here, I did a quick refactoring into general code:

https://github.com/jgunthorpe/linux/commits/for-baolu/

You just need to provide a used function to compute which bits HW is
not ignoring and a sync function to push the invalidation command. It
will take care of all sequencing needs for all possible new/old
combinations.

Then delete the replace/not replace split in the code too.

Jason