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Re: [PATCH] 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 13:19:58 +0100
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
- Delivery-date: Tue, 07 May 2024 12:20:12 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 07/05/2024 12:50 pm, Roger Pau Monné wrote:
> On Tue, May 07, 2024 at 12:29:57PM +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 host policy's feat.max_subleaf to the hightest number Xen knows
>> about, similarly to how we extend extd.max_leaf for LFENCE_DISPATCH. This
>> will allow VMs with a higher feat.max_subleaf than strictly necessary to
>> migrate in.
> Seeing what we do for max_extd_leaf, shouldn't we switch to doing what
> you propose for feat.max_subleaf to max_extd_leaf also?
>
> To allow migration between hosts that have 0x80000021.eax and hosts
> that don't have such extended leaf.
>
> cpu_has_lfence_dispatch kind of does that, but if lfence cannot be
> made serializing then the max extended leaf is not expanded. And we
> should also likely account for more feature leafs possibly appearing
> after 0x80000021?
On second thoughts, this adjustment ought to be in the max policies only.
It's slightly different to LFENCE_DISPATCH, in that we don't actually
have any set bits in those leaves.
I'll do a different patch.
~Andrew
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