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

["Radim Krčmář" <rkrcmar@redhat.com>]

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.

>>> 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?

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.