linux-kernel - Re: [PATCH rc v1 1/4] iommu/arm-smmu-v3: Add ignored bits to fix STE update sequence

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Message-ID: <aTXW5X4fAiEHAy6V@nvidia.com>
Date: Sun, 7 Dec 2025 11:35:01 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <will@...nel.org>, <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>
Subject: Re: [PATCH rc v1 1/4] iommu/arm-smmu-v3: Add ignored bits to fix STE
 update sequence

On Sun, Dec 07, 2025 at 12:09:10PM -0400, Jason Gunthorpe wrote:
> On Sat, Dec 06, 2025 at 08:37:30PM -0800, Nicolin Chen wrote:
> > > Then ignored should be adjusted by the used: Only if both used are 1
> > > should the bit become ignored. Otherwise we can rely on which ever
> > > used is 0 to generate the hitless update.
> > 
> > Hmm, not sure why it has to be both used.
> 
> Thats the only case that causes an issue, if only one is used then
> there is no need to perform a breaking update.
> 
> If cur_used is 0 then the bit will be set in the first update, if
> target_used is 0 then the bit will be set during the last update.

Ah, I see.

> > >                 /* Bits can change because they are not currently being used */
> > > +               cur_used[i] &= ~ignored[i];
> > >                 unused_update[i] = (entry[i] & cur_used[i]) |
> > >                                    (target[i] & ~cur_used[i]);
> > 
> > If one of ignored bits is set in entry[i] but unset in target[i],
> > the unused_update will first mask it away, resulting in an extra
> > unnecessary update (though it's still hitless).
> 
> Yes, this is how it has always worked. The point is to leave the
> existing the same not try to optimize it using ignored.

OK. Let's leave it and ask the test case to expect 3 v.s. 2.

> > One more change that we need is at the last equation:
> > -		if ((unused_update[i] & target_used[i]) != target[i])
> > +		if ((unused_update[i] & target_used[i] & ~ignored[i]) !=
> > +		    (target[i] & ~ignored[i]))
> > 
> > Either side might have the ignored bits, so we have to suppress
> > ignored on both sides, which is required in the similar routine
> > in arm_smmu_entry_differs_in_used_bits() of the kunit code.
> 
> The only way ignored is set is if both sides have it set and then we
> update the bit in the firsy cycle meaning unused_update must have the
> final value. There is no need to mask target since it will match. Not
> changing this line is a big part of what makes this appealing because
> it keeps the logic straightforward, in case ignored is used we shift
> the update always to the first cycle then everything else is the same.

The reason that I changed this is because the kunit tests failed
in arm_smmu_entry_differs_in_used_bits() when running the nested
cases:

STE initial value: 
    f800f0f0f0f0f0ef 00001000180800d5
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE used bits: 
    f80fffffffffffff 00003000fa0800ff
    065fffff0000ffff 000ffffffffffff0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE target value: 
    000000000000000d 0000100000000000
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE used bits: 
    000000000000000f 0000300032080000
    065fffff0000ffff 000ffffffffffff0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE value is now set to: 
    f800f0f0f0f0f0ef 00001000080000d5
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
    # arm_smmu_v3_write_ste_test_nested_s1dssbypass_to_s1bypass: EXPECTATION FAILED at drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-test.c:92
                   Expected arm_smmu_entry_differs_in_used_bits( test_writer->entry, entry_used_bits, test_writer->init_entry, ignored, 8) && arm_smmu_entry_differs_in_used_bits( test_writer->entry, entry_used_bits, test_writer->target_entry, ignored, 8) to be false, but is true
STE value is now set to: 
    000000000000000d 00001000080000d5
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE value is now set to: 
    000000000000000d 0000100000000000
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
    not ok 21 arm_smmu_v3_write_ste_test_nested_s1dssbypass_to_s1bypass

STE initial value: 
    000000000000000d 0000100000000000
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE used bits: 
    000000000000000f 0000300032080000
    065fffff0000ffff 000ffffffffffff0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE target value: 
    f800f0f0f0f0f0ef 00001000180800d5
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE used bits: 
    f80fffffffffffff 00003000fa0800ff
    065fffff0000ffff 000ffffffffffff0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
STE value is now set to: 
    000000000000000d 00001000180800d5
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
    # arm_smmu_v3_write_ste_test_nested_s1bypass_to_s1dssbypass: EXPECTATION FAILED at drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-test.c:92
                   Expected arm_smmu_entry_differs_in_used_bits( test_writer->entry, entry_used_bits, test_writer->init_entry, ignored, 8) && arm_smmu_entry_differs_in_used_bits( test_writer->entry, entry_used_bits, test_writer->target_entry, ignored, 8) to be false, but is true
STE value is now set to: 
    f800f0f0f0f0f0ef 00001000180800d5
    0449b6c400000000 000dbeefdeadbee0
    0000000000000000 0000000000000000
    0000000000000000 0000000000000000
    not ok 22 arm_smmu_v3_write_ste_test_nested_s1bypass_to_s1dssbypass

Any thought?

Thanks
Nicolin