Date: Fri, 11 Aug 2017 15:35:31 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc: David Hildenbrand <david@redhat.com>,
        Lan Tianyu <tianyu.lan@intel.com>, pbonzini@redhat.com,
        tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM/x86: Increase max vcpu number to 352
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On Fri, Aug 11, 2017 at 03:00:20PM +0200, Radim Krčmář wrote:
> 2017-08-11 10:11+0200, David Hildenbrand:
> > 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.
> 
> Yeah, the only issue until around 2^20 (when we reach the maximum of
> logical x2APIC addressing) should be the size of per-VM arrays when only
> few VCPUs are going to be used.

Migration with 352 CPUs all being busy dirtying memory and also poking
at various I/O ports (say all of them dirtying the VGA) is no problem?


> 
> >>> 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.


By different platforms you mean q35 vs the older one, and such?
Not whether the underlaying accelerator is tcg, Xen, KVM, or bHyve?

What I was trying to understand whether it makes even sense for
the platforms to have such limits in the first place - and instead the
accelerators should be the ones setting it?


> >> 
> >> 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?
> 
> That would be great.
> 
> > commit 682f732ecf7396e9d6fe24d44738966699fae6c0
> > Author: Radim Krčmář <rkrcmar@redhat.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 :] ).
> 
> Eh, I forgot about them ...  I didn't like the dynamic allocation as we
> would need to protect the memory, which would result in a much bigger
> changeset, or fragile macros.
> 
> I can't recall the disgust now, so I'll send a RFC with the dynamic
> version to see how it turned out.
> 
> Thanks.