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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-06-05 at 15:37 +0100, Julien Grall wrote:
> 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".

Thanks, they really buried the lead on that one, the section introducing
affinity hierarchy doesn't even hint at this limit!

> 
> 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,
> 



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