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Re: [PATCH] x86/APIC: Remove x2APIC pure cluster mode
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 24 Sep 2024 19:20:44 +0100
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Community Manager <community.manager@xxxxxxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Matthew Barnes <matthew.barnes@xxxxxxxxx>
- Delivery-date: Tue, 24 Sep 2024 18:20:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 24/09/2024 6:10 pm, Roger Pau Monné wrote:
> On Tue, Sep 24, 2024 at 06:21:47PM +0200, Jan Beulich wrote:
>> On 24.09.2024 18:14, Roger Pau Monné wrote:
>>> On Tue, Sep 24, 2024 at 04:27:36PM +0100, Andrew Cooper wrote:
>>>> On 24/09/2024 4:10 pm, Roger Pau Monné wrote:
>>>>> On Mon, Sep 23, 2024 at 03:35:59PM +0100, Matthew Barnes wrote:
>>>>>> With the introduction of mixed x2APIC mode (using cluster addressing for
>>>>>> IPIs and physical for external interrupts) the use of pure cluster mode
>>>>>> doesn't have any benefit.
>>>>>>
>>>>>> Remove the mode itself, leaving only the code required for logical
>>>>>> addressing when sending IPIs.
>>>>>>
>>>>>> Implements: https://gitlab.com/xen-project/xen/-/issues/189
>>>> We use the Resolves: tag for this. Can fix on commit.
>>>>
>>>>> There's at least one extra bit which I would also like to see removed,
>>>>> either in this patch, or as following patch.
>>>>>
>>>>> In struct arch_irq_desc we have 3 cpumasks: cpu_mask, old_cpu_mask and
>>>>> pending_mask. After dropping cluster mode for external interrupts,
>>>>> those fields could become integers AFACT, as now interrupts can only
>>>>> target a single CPU opposed to a logical CPU set.
>>>> A separate patch for sure, but that sounds like a great improvement.
>>> Oh, there are quite some fields of struct genapic that are not needed
>>> anymore, since both implementations set it to the same function. It
>>> would be helpful to prune them.
>> Pruning where possible - yes. But "both" won't cover it, as we have 4
>> instances of the struct (not just the two x2apic ones).
> Yeah, realized that afterwards, we still have the xAPIC flat mode,
> which is using logical delivery mode, but target a single CPU. So
> getting rid of the cpumask in arch_irq_desc seem possible, however
> there might be nothing to prune in struct genapic.
Logical delivery mode for external interrupts in xAPIC is just as
broken/useless as the code we've just deleted.
If that's the only thing in the way of more cleanup, we delete it too.
~Andrew
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