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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
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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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