Re: [PATCH rc v5 1/4] iommu/arm-smmu-v3: Add update_safe bits to fix STE update sequence

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jan 13, 2026 at 12:29:11PM -0800, Nicolin Chen wrote:
> On Tue, Jan 13, 2026 at 12:12:53PM -0400, Jason Gunthorpe wrote:
> > On Tue, Jan 13, 2026 at 03:05:52PM +0000, Will Deacon wrote:
> > > I suppose we shouldn't ever see the case that they both have S2S, but
> > > that's fine.
> > 
> > If they both have S2S then it works correctly? Any S2S forces EATS to
> > follow the normal rules.
> > 
> > > The spec also suggests that there's an additional illegal STE case w/
> > > split-stage ATS (EATS_S1CHK) if Config != S1+S2.
> > 
> > The driver doesn't support that either..
> > 
> > It is fixed by checking if new EATS is valid under old config and old
> > EATS valid under new config.
> > 
> > Also to support S1CHK someday we cannot allow the hypervisor to leave
> > S1_S2 and go to S2, since the HW can't deal with that...
> 
> Perhaps the safe bits should only have EATS_TRANS, excluding S1CHK:
> 
> --------------------------------------------------------------------------
> +        * When an STE changes EATS_TRANS, the sequencing code in the attach
> +        * logic already will have the PCI cap for ATS disabled. Thus at this
> +        * moment we can expect that the device will not generate ATS queries
> +        * and so we don't care about the sequencing of EATS. The purpose of
> +        * EATS_TRANS is to protect the system from hostile untrusted devices
> +        * that issue ATS when the PCI config space is disabled. However, if
> +        * EATS_TRANS is being changed, then we must have already trusted the
> +        * device as the EATS_TRANS security block is being disabled.
> +        *
> +        *  Note: now the EATS_TRANS update is moved to the first entry_set().
> +        *  Changing S2S and EATS might transiently result in S2S=1 and EATS=1
> +        *  which is a bad STE (see "5.2 Stream Table Entry"). In such a case,
> +        *  we can't do a hitless update. Also, STRTAB_STE_1_EATS_S1CHK should
> +        *  not be added to the safe bits because it relies on CFG[2:0]=0b111,
> +        *  to prevent another bad STE.
>          */
> -       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.

Thanks,
Jason