Re: [PATCH] x86: Raise the hard VCPU count limit

[Sasha Levin <levinsasha928@gmail.com>]

On Wed, 2011-07-13 at 16:30 +0300, Avi Kivity wrote:
> On 07/09/2011 03:25 PM, Sasha Levin wrote:
> > The patch raises the hard limit of VCPU count to 1024.
> >
> > This will allow developers to easily work on scalability
> > and will allow users to test high VCPU setups easily without
> > patching the kernel.
> >
> > To prevent possible issues with current setups, KVM_CAP_NR_VCPUS
> > now returns the recommended VCPU limit (which is still 64) - this
> > should be a safe value for everybody, while a new KVM_CAP_MAX_VCPUS
> > returns the hard limit which is now 1024.
> >
> 
> Can 1024 vcpus even work without interrupt remapping?
> 

I'm not sure. I've successfully tried it with 255 vcpus.

> Looks like the patch will break coalesced mmio:
> 
> static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev,
>                     gpa_t addr, int len)
> {
>      struct kvm_coalesced_mmio_zone *zone;
>      struct kvm_coalesced_mmio_ring *ring;
>      unsigned avail;
>      int i;
> 
>      /* Are we able to batch it ? */
> 
>      /* last is the first free entry
>       * check if we don't meet the first used entry
>       * there is always one unused entry in the buffer
>       */
>      ring = dev->kvm->coalesced_mmio_ring;
>      avail = (ring->first - ring->last - 1) % KVM_COALESCED_MMIO_MAX;
>      if (avail < KVM_MAX_VCPUS) {
>          /* full */
>          return 0;
>      }
> 
> 

I don't quite understand what KVM_MAX_VCPUS has to do with that if ().
Shouldn't it check whether theres more than one buffer between first and
last? What role does KVM_MAX_VCPUS play there?

-- 

Sasha.