linux-kernel - Re: [PATCH] KVM/x86: Increase max vcpu number to 352

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Message-ID: <aab6d4cc-770c-4c49-c587-ff693e23388f@redhat.com>
Date:   Fri, 11 Aug 2017 10:11:01 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Lan Tianyu <tianyu.lan@...el.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        rkrcmar@...hat.com
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM/x86: Increase max vcpu number to 352

On 11.08.2017 09:49, Lan Tianyu wrote:
> Hi Konrad:
> 	Thanks for your review.
> 
> On 2017年08月11日 01:50, Konrad Rzeszutek Wilk wrote:
>> On Thu, Aug 10, 2017 at 06:00:59PM +0800, Lan Tianyu wrote:
>>> Intel Xeon phi chip will support 352 logical threads. For HPC usage
>>> case, it will create a huge VM with vcpu number as same as host cpus. This
>>> patch is to increase max vcpu number to 352.
>>
>> Why not 1024 or 4096?
> 
> This is on demand. We can set a higher number since KVM already has
> x2apic and vIOMMU interrupt remapping support.
> 
>>
>> Are there any issues with increasing the value from 288 to 352 right now?
> 
> No found.
> 
>>
>> Also perhaps this should be made in an Kconfig entry?
> 
> That will be anther option but I find different platforms will define
> different MAX_VCPU. If we introduce a generic Kconfig entry, different
> platforms should have different range.
> 
> Radim & Paolo, Could you give some input? In qemu thread, we will set
> max vcpu to 8192 for x86 VM. In KVM, The length of vcpu pointer array in
> struct kvm and dest_vcpu_bitmap in kvm_irq_delivery_to_apic() are
> specified by KVM_MAX_VCPUS. Should we keep align with Qemu?
> 

commit 682f732ecf7396e9d6fe24d44738966699fae6c0
Author: Radim Krčmář <rkrcmar@...hat.com>
Date:   Tue Jul 12 22:09:29 2016 +0200

    KVM: x86: bump MAX_VCPUS to 288

    288 is in high demand because of Knights Landing CPU.
    We cannot set the limit to 640k, because that would be wasting space.

I think we want to keep it small as long as possible. I remember a patch
series from Radim which would dynamically allocate memory for these
arrays (using a new VM creation ioctl, specifying the max # of vcpus).
Wonder what happened to that (I remember requesting a simply remalloc
instead of a new VM creation ioctl :] ).

-- 

Thanks,

David