Re: [Xen-devel] [PATCH V6 00/10] Support more than 8 vcpus on arm64 with GICv3

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 05/06/15 15:08, Ian Campbell wrote:
> On Mon, 2015-06-01 at 20:56 +0800, Chen Baozi wrote:
>> From: Chen Baozi <baozich@xxxxxxxxx>
>>
>> Currently the number of vcpus on arm64 with GICv3 is limited up to 8 due
>> to the fixed size of redistributor mmio region.
> 
> Are we talking about only guests here or are hosts impacted too somehow?

Guest only. The GICv3 driver should already supports 128 CPUs (though
never tested).

>>  Increasing the size
>> makes the number expand to 16 because of AFF0 restriction on GICv3.
> 
> Can you give a reference for this please? "AFF0" doesn't appear anywhere
> in my gic v3 spec, and "4.2.2 Interrupt Routing in GICv3 Systems"
> implies 256 processors at each affinity level (which is what I
> expected).

5.7.29 ICC_SGI0R_EL1, ICC_SGI1R_EL1 and ICC_ASGI1R_EL1:

"Note: this restricts distribution of SGIs to the first 16 processors of
an affinity 1 cluster".

Therefore any CPU using AFF0 >= 16 would never receive SGI.

FWIW, Linux is shouting loud when the AFF0 is too high. Xen does the
same thing in the GICv3 drivers.


Regards,

-- 
Julien Grall

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