linux-kernel - Re: [PATCH] KVM: arm/arm64: vgic: Use a single IO device per redistributor

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Date:   Tue, 27 Aug 2019 15:49:29 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Auger Eric <eric.auger@...hat.com>, <eric.auger.pro@...il.com>,
        <maz@...nel.org>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>, <kvmarm@...ts.cs.columbia.edu>
CC:     <zhang.zhanghailiang@...wei.com>, <wanghaibin.wang@...wei.com>,
        <james.morse@....com>, <qemu-arm@...gnu.org>,
        <julien.thierry.kdev@...il.com>, <suzuki.poulose@....com>,
        <peter.maydell@...aro.org>, <andre.przywara@....com>
Subject: Re: [PATCH] KVM: arm/arm64: vgic: Use a single IO device per
 redistributor

Hi Eric,

Thanks for this patch!

On 2019/8/24 1:52, Auger Eric wrote:
> Hi Zenghui, Marc,
> 
> On 8/23/19 7:33 PM, Eric Auger wrote:
>> At the moment we use 2 IO devices per GICv3 redistributor: one
                                                              ^^^
>> one for the RD_base frame and one for the SGI_base frame.
   ^^^
>>
>> Instead we can use a single IO device per redistributor (the 2
>> frames are contiguous). This saves slots on the KVM_MMIO_BUS
>> which is currently limited to NR_IOBUS_DEVS (1000).
>>
>> This change allows to instantiate up to 512 redistributors and may
>> speed the guest boot with a large number of VCPUs.
>>
>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> 
> I tested this patch with below kernel and QEMU branches:
> kernel: https://github.com/eauger/linux/tree/256fix-v1
> (Marc's patch + this patch)
> https://github.com/eauger/qemu/tree/v4.1.0-256fix-rfc1-rc0
> (header update + kvm_arm_gic_set_irq modification)

I also tested these three changes on HiSi D05 (with 64 pcpus), and yes,
I can get a 512U guest to boot properly now.

Tested-by: Zenghui Yu <yuzenghui@...wei.com>

> On a machine with 224 pcpus, I was able to boot a 512 vcpu guest.
> 
> As expected, qemu outputs warnings:
> 
> qemu-system-aarch64: warning: Number of SMP cpus requested (512) exceeds
> the recommended cpus supported by KVM (224)
> qemu-system-aarch64: warning: Number of hotpluggable cpus requested
> (512) exceeds the recommended cpus supported by KVM (224)
> 
> on the guest: getconf _NPROCESSORS_ONLN returns 512
> 
> Then I have no clue about what can be expected of such overcommit config
> and I have not further exercised the guest at the moment. But at least
> it seems to boot properly. I also tested without overcommit and it seems
> to behave as before (boot, migration).
> 
> I still need to look at the migration of > 256vcpu guest at qemu level.

Let us know if further tests are needed.


Thanks,
zenghui