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Re: [PATCH 2/6] x86/x2apic: use physical destination mode by default
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 23 Jun 2022 16:49:38 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 23 Jun 2022 14:49:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23.06.2022 10:24, Roger Pau Monne wrote:
> Using cluster mode by default greatly limits the amount of vectors
> available, as then vector space is shared amongst all the CPUs in the
> logical cluster.
>
> This can lead to vector shortage issues on boxes with not a huge
> amount of CPUs but with a non-trivial amount of devices, there are
> reports of boxes with 32 CPUs (2 logical clusters, and thus only 414
> dynamic vectors) that run out of vectors and fail to setup interrupts
> for dom0.
>
> This could be considered as a regression when switching from xAPIC
> mode, as when using xAPIC only physical mode is supported.
When using more than 8 CPUs.
You also don't mention the downside (higher IPI send effort) at all.
Jan
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