Re: [PATCH] KVM: PPC: Add KVM_CAP_NR_VCPUS and KVM_CAP_MAX_VCPUS

[Sasha Levin <levinsasha928@gmail.com>]

On Tue, 2011-12-20 at 16:17 +0100, Alexander Graf wrote:
> On 08.12.2011, at 03:55, Matt Evans wrote:
> 
> > PPC KVM lacks these two capabilities, and as such a userland system must assume
> > a max of 4 VCPUs (following api.txt).  With these, a userland can determine
> > a more realistic limit.
> > 
> > Signed-off-by: Matt Evans <matt@ozlabs.org>
> > ---
> > 
> > Alex: For when you're back in civilisation -- the kvmtool/PPC stuff will be
> > limited to 4 VCPUs until the kernel returns something for these caps.
> > 
> > Cheers, Matt
> > 
> > arch/powerpc/kvm/powerpc.c |   15 +++++++++++++++
> > 1 files changed, 15 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> > index 7c7220c..3f7219d 100644
> > --- a/arch/powerpc/kvm/powerpc.c
> > +++ b/arch/powerpc/kvm/powerpc.c
> > @@ -245,6 +245,21 @@ int kvm_dev_ioctl_check_extension(long ext)
> > 			r = 2;
> > 		break;
> > #endif
> > +	case KVM_CAP_NR_VCPUS:
> > +		/*
> > +		 * Recommending a number of CPUs is somewhat arbitrary; we return the number of present
> > +		 * CPUs for -HV (since a host will have secondary threads "offline"), and for other KVM
> > +		 * implementations just count online CPUs.
> > +		 */
> > +#ifdef CONFIG_KVM_BOOK3S_64_HV
> > +		r = num_present_cpus();
> > +#else
> > +		r = num_online_cpus();
> > +#endif
> 
> That will essentially restrict us to not allow overcommitting when in the scope of a single VM. Is that what we want? You could easily run a 32-way guest on a 4-way host, even with _HV.
> 
> Maybe some really big number makes more sense here.

These two caps are defined pretty well on x86:

KVM_CAP_MAX_VCPUS - Absolute possible number of vcpus we can squeeze in
a single guest due to some technical limitation. On x86 it's limited to
254 due to not supporting IRQ remapping.

KVM_CAP_NR_VCPUS - This is actually an arbitrary number which reflects
the point at which adding more vcpus over this number will actually
cause a performance hit.

-- 

Sasha.