Re: [PATCH] x86, apic: Enable x2APIC physical when cpu < 256 native

[Ingo Molnar]

* Youquan Song <youquan.song@xxxxxxxxx> wrote:

> x2APIC extends APICID from 8 bits to 32 bits, but the device interrupt 
> routed from IOAPIC or delivered in MSI mode will keep 8 bits destination 
> APICID. In order to support x2APIC, the VT-d interrupt remapping is 
> introduced to translate the destination APICID to 32 bits in x2APIC mode 
> and keep the device compatible in this way.
> 
> x2APIC support both logical and physical mode in destination mode.  In 
> logical destination mode, the 32 bits Logical APICID has 2 sub-fields:
>  16 bits cluster ID and 16 bits logical ID within the cluster and it is 
> required VT-d interrupt remapping in x2APIC cluster mode. In physical 
> destination mode, the 8 bits physical id is compatible with 32 bits 
> physical id when CPU number < 256. When interrupt remapping 
> initialization fail on platform with CPU number < 256, current kernel 
> only enables x2APIC physical mode in virutalization environment, while 
> we also can enable x2APIC physcial mode in native kernel this situation, 
> and the device interrupt will use 8 bits destination APICID in physical 
> mode and be compatible with x2APIC physical when < 256 CPUs.
>  
> So we can benefit from x2APIC vs xAPIC MMIO:
>  - x2APIC MSR read/write is faster than xAPIC mmio
>  - x2APIC only ICR write to deliver interrupt without polling ICR deliver 
>    status bit and xAPIC need poll to read ICR deliver status bit.
>  - x2APIC 64 bits ICR access instead of xAPIC two 32 bits access.

That looks interesting. How many systems are affected by this change in 
practice? Have you tested it on affected hardware?

Thanks,

	Ingo
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