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

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Message-ID: <eeab7d3c-ede1-4e55-a55a-f8b7344ca164@linux.intel.com>
Date: Wed, 14 Jan 2026 14:03:58 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...dia.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 1/13/26 23:05, Jason Gunthorpe wrote:
> 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.

It's always good to generate common library code to avoid boilerplate
code and different behaviors across multiple drivers. I'll try to
integrate this into the next version. Thanks for the help!

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

Thanks,
baolu