Re: [Xen-devel] [PATCH V5 10/10] xen/arm64: increase MAX_VIRT_CPUS to 128 on arm64

[Chen Baozi <baozich@xxxxxxxxx>]

On Sun, May 31, 2015 at 07:21:22PM +0100, Julien Grall wrote:
> Hi Chen,
> 
> On 31/05/2015 16:37, Chen Baozi wrote:
> >
> >>On May 31, 2015, at 21:40, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
> >>
> >>Hi Chen,
> >>
> >>On 30/05/2015 12:07, Chen Baozi wrote:
> >>>From: Chen Baozi <baozich@xxxxxxxxx>
> >>>
> >>>GIC-500 supports up to 128 cores in a single SoC. Increase MAX_VIRT_CPUS
> >>>to 128 on arm64.
> >>
> >>Where did you find this restriction? AFAICT the changes you made in the 
> >>vGICv3 driver allow us to use up to 4096 CPUs.
> >
> >In ARM DDI0516B (ARM CoreLinkTM GIC-500 Generic Interrupt Controller
> >Technical Reference Manual):
> >
> >1.1 About the GIC-500
> >
> >The GIC-500 is a build-time configurable interrupt controller that supports 
> >up to 128 cores.
> >...
> >
> >And GIC-500 is the only GICv3 implementation as far as I know.
> 
> The only implementation hardware ;). The vGICv3 in both KVM and Xen are 2
> other implementations but software.
> 
> For instance, with your series a cluster can use up to 16 cores but the
> GIC-500 is only supporting up to 8 cores...

Well, if 4096 is a acceptable value, it will cost 512M address space for GICR_*
(We need to chagne patch #1 too). Although we do have enough space for GICR_*
before it reaches the GNTTAB region, I don't think it is a good idea to
increase MAX_VIRT_CPUS to such a big one.

And I've also check the old value of MAX_VIRT_CPUS, it used to be 128 before
aa25a61.

Or do you have a better suggestion?

Cheers,

Baozi.

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