linux-kernel - Re: [PATCH rc v5 1/4] iommu/arm-smmu-v3: Add update

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Message-ID: <aWkmXO2GBj00AVwy@willie-the-truck>
Date: Thu, 15 Jan 2026 17:39:40 +0000
From: Will Deacon <will@...nel.org>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: Jason Gunthorpe <jgg@...dia.com>, 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, smostafa@...gle.com
Subject: Re: [PATCH rc v5 1/4] iommu/arm-smmu-v3: Add update_safe bits to fix
 STE update sequence

On Thu, Jan 15, 2026 at 08:34:43AM -0800, Nicolin Chen wrote:
> On Thu, Jan 15, 2026 at 12:29:19PM -0400, Jason Gunthorpe wrote:
> > On Thu, Jan 15, 2026 at 08:25:05AM -0800, Nicolin Chen wrote:
> > > On Thu, Jan 15, 2026 at 09:11:51AM -0400, Jason Gunthorpe wrote:
> > > > On Tue, Jan 13, 2026 at 04:51:12PM -0400, Jason Gunthorpe wrote:
> > > > > > -       safe_bits[1] |= cpu_to_le64(STRTAB_STE_1_EATS);
> > > > > > +       if (!((cur[2] | target[2]) & cpu_to_le64(STRTAB_STE_2_S2S)))
> > > > > > +               safe_bits[1] |= cpu_to_le64(
> > > > > > +                       FIELD_PREP(STRTAB_STE_1_EATS, STRTAB_STE_1_EATS_TRANS));
> > > > > > --------------------------------------------------------------------------
> > > > > > 
> > > > > > @will, does this look good to you? I can send a v7 with this.
> > > > > 
> > > > > That is an easy way to address Will's observation, makes sense to me.
> > > > 
> > > > Ah, but it looks like it can generate an errant view of a EATS that is
> > > > neither old or new. Ie value 3, reserved.
> > > > 
> > > > I think you should just check if old or new has EATS bit 1 set:
> > > > 
> > > > if (!((cur[2] | target[2]) & cpu_to_le64(STRTAB_STE_2_S2S)) &&
> > > >     !((cur[1] | target[1]) & cpu_to_le64(FIELD_PREP(STRTAB_STE_1_EATS, 2))))
> > > > 
> > > > Which the current driver never does..
> > > 
> > > The EATS field is completely controlled by the driver. So, we are
> > > safe for now, right?
> > > 
> > > Should we add this when the driver has the actual support for the
> > > split stage thing?
> > 
> > If we have figured it out now I would add it because it would be a big
> > leap to think the next person will remember about this detail..
> > 
> > But yes, this and the S2S thing don't effect the driver as it is now,
> > it is just doing work to help future people.
> 
> OK. Let's add that.
> 
> I will send the v7 by the end of the day. Hopefully, Will is okay
> with all of these..

Sounds about right but I'll wait and see what you post.

Will