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Re: [PATCH v2] x86/gen-cpuid: correct cycle detection
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 1 Sep 2025 13:02:50 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 01 Sep 2025 11:03:07 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 01.09.2025 12:31, Andrew Cooper wrote:
> On 01/09/2025 9:56 am, Jan Beulich wrote:
>> With the processing done linearly (rather than recursively), checking
>> whether any of the features was previously seen is wrong: That would
>> e.g. trigger for this simple set of dependencies
>>
>> X: [A, B]
>> A: [C]
>> B: [C]
>>
>> (observed in reality when making AMX-AVX512 dependent upon both
>> AMX-TILE and AVX512F, causing XSAVE to see AMX-AVX512 twice in its list
>> of dependents). But checking the whole accumulated set also isn't
>> necessary - just checking the feature we're processing dependents of is
>> sufficient. We may detect a cycle later that way, but we still will
>> detect it. What we need to avoid is adding a feature again when we've
>> already seen it.
>>
>> As a result, seeding "seen[]" with "feat" isn't necessary anymore.
>>
>> Fixes: fe4408d180f4 ("xen/x86: Generate deep dependencies of features")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, with one further
> minor adjustment.
Thanks.
>> --- a/xen/tools/gen-cpuid.py
>> +++ b/xen/tools/gen-cpuid.py
>> @@ -379,14 +379,17 @@ def crunch_numbers(state):
>>
>> f = to_process.pop(0)
>>
>> + if f == feat:
>> + raise Fail("ERROR: Cycle found when processing %s" % \
>
> No need for the \ here.
Okay, but then why is there one in the commented out code you touch in the
other patch?
Jan
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