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

[Baolu Lu]

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