lkml.org 
[lkml]   [2021]   [Nov]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS
    On Fri, Nov 12, 2021 at 10:51:10AM +0100, Vitaly Kuznetsov wrote:
    > Marc Zyngier <maz@kernel.org> writes:
    >
    > > Hi Vitaly,
    > >
    > > On 2021-11-11 16:27, Vitaly Kuznetsov wrote:
    > >> It doesn't make sense to return the recommended maximum number of
    > >> vCPUs which exceeds the maximum possible number of vCPUs.
    > >>
    > >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
    > >> ---
    > >> arch/arm64/kvm/arm.c | 7 ++++++-
    > >> 1 file changed, 6 insertions(+), 1 deletion(-)
    > >>
    > >> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
    > >> index 7838e9fb693e..391dc7a921d5 100644
    > >> --- a/arch/arm64/kvm/arm.c
    > >> +++ b/arch/arm64/kvm/arm.c
    > >> @@ -223,7 +223,12 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm,
    > >> long ext)
    > >> r = 1;
    > >> break;
    > >> case KVM_CAP_NR_VCPUS:
    > >> - r = num_online_cpus();
    > >> + if (kvm)
    > >> + r = min_t(unsigned int, num_online_cpus(),
    > >> + kvm->arch.max_vcpus);
    > >> + else
    > >> + r = min_t(unsigned int, num_online_cpus(),
    > >> + kvm_arm_default_max_vcpus());
    > >> break;
    > >> case KVM_CAP_MAX_VCPUS:
    > >> case KVM_CAP_MAX_VCPU_ID:
    > >
    > > This looks odd. This means that depending on the phase userspace is
    > > in while initialising the VM, KVM_CAP_NR_VCPUS can return one thing
    > > or the other.
    > >
    > > For example, I create a VM on a 32 CPU system, NR_VCPUS says 32.
    > > I create a GICv2 interrupt controller, it now says 8.
    > >
    > > That's a change in behaviour that is visible by userspace
    >
    > Yes, I realize this is a userspace visible change. The reason I suggest
    > it is that logically, it seems very odd that the maximum recommended
    > number of vCPUs (KVM_CAP_NR_VCPUS) can be higher, than the maximum
    > supported number of vCPUs (KVM_CAP_MAX_VCPUS). All userspaces which use
    > this information somehow should already contain some workaround for this
    > case. (maybe it's a rare one and nobody hit it yet or maybe there are no
    > userspaces using KVM_CAP_NR_VCPUS for anything besides complaining --
    > like QEMU).
    >
    > I'd like KVM to be consistent across architectures and have the same
    > (similar) meaning for KVM_CAP_NR_VCPUS.

    KVM_CAP_NR_VCPUS seems pretty useless if we just want to tell userspace
    the same thing it can get with _SC_NPROCESSORS_ONLN. In fact, if userspace
    knows something we don't about the future onlining of some pcpus, then
    maybe userspace would prefer to check _SC_NPROCESSORS_CONF.

    >
    > > which I'm keen on avoiding. I'd rather have the kvm and !kvm cases
    > > return the same thing.
    >
    > Forgive me my (ARM?) ignorance but what would it be then? If we go for
    > min(num_online_cpus(), kvm_arm_default_max_vcpus()) in both cases, cat
    > this can still go above KVM_CAP_MAX_VCPUS after vGIC is created?

    So the GIC version case looks like the type of thing that could make
    KVM_CAP_NR_VCPUS useful, i.e. being able to tell userspace a maximum
    number of vcpus supported for a given configuration. However, even
    in that case the concept of "recommended" number doesn't make sense,
    because, for the GICv2 example, a VM cannot configure more than 8 VCPUs,
    so it's a real limit, not a recommendation. Maybe KVM_CAP_NR_VCPUS should
    just be left alone, but deprecated, and, if there's need, a new CAP could
    be created for a config-vcpu-max.

    Thanks,
    drew

    \
     
     \ /
      Last update: 2021-11-12 11:41    [W:4.254 / U:0.000 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site