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

[Nicolin Chen <nicolinc@nvidia.com>]

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.

Thanks
Nicolin