On 20.12.2011, at 16:32, Sasha Levin wrote: > 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@xxxxxxxxxx> >>> --- >>> >>> 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. Ah cool - then it's all fine and shiny. Thanks for the reminder! :) Applied the patch to kvm-ppc-next (with 80 character line limit fixed). Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
