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Re: [PATCH v2] x86/cpu-policy: Fix migration from Ice Lake to Cascade Lake
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 7 May 2024 15:31:19 +0100
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
- Delivery-date: Tue, 07 May 2024 14:31:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 07/05/2024 3:24 pm, Roger Pau Monné wrote:
> On Tue, May 07, 2024 at 02:45:40PM +0100, Andrew Cooper wrote:
>> Ever since Xen 4.14, there has been a latent bug with migration.
>>
>> While some toolstacks can level the features properly, they don't shink
>> feat.max_subleaf when all features have been dropped. This is because
>> we *still* have not completed the toolstack side work for full CPU Policy
>> objects.
>>
>> As a consequence, even when properly feature levelled, VMs can't migrate
>> "backwards" across hardware which reduces feat.max_subleaf. One such example
>> is Ice Lake (max_subleaf=2 for INTEL_PSFD) to Cascade Lake (max_subleaf=0).
>>
>> Extend the max policies feat.max_subleaf to the hightest number Xen knows
>> about, but leave the default policies matching the host. This will allow VMs
>> with a higher feat.max_subleaf than strictly necessary to migrate in.
>>
>> Eventually we'll manage to teach the toolstack how to avoid creating such VMs
>> in the first place, but there's still more work to do there.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Thanks.
>
> Even if we have just found one glitch with PSFD and Ice Lake vs
> Cascade Lack, wouldn't it be safer to always extend the max policies
> max leafs and subleafs to match the known array sizes?
This is the final max leaf (containing feature information) to gain
custom handling, I think?
~Andrew
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