On Wed, 11 Sep 2019 12:30:48 +1000
David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Sep 10, 2019 at 06:49:34PM +0200, Greg Kurz wrote:
> > Each vCPU of a VM allocates a XIVE VP in OPAL which is associated with
> > 8 event queue (EQ) descriptors, one for each priority. A POWER9 socket
> > can handle a maximum of 1M event queues.
> >
> > The powernv platform allocates NR_CPUS (== 2048) VPs for the hypervisor,
> > and each XIVE KVM device allocates KVM_MAX_VCPUS (== 2048) VPs. This means
> > that on a bi-socket system, we can create at most:
> >
> > (2 * 1M) / (8 * 2048) - 1 == 127 XIVE or XICS-on-XIVE KVM devices
> >
> > ie, start at most 127 VMs benefiting from an in-kernel interrupt controller.
> > Subsequent VMs need to rely on much slower userspace emulated XIVE device in
> > QEMU.
> >
> > This is problematic as one can legitimately expect to start the same
> > number of mono-CPU VMs as the number of HW threads available on the
> > system (eg, 144 on Witherspoon).
> >
> > I'm not aware of any userspace supporting more that 1024 vCPUs. It thus
> > seem overkill to consume that many VPs per VM. Ideally we would even
> > want userspace to be able to tell KVM about the maximum number of vCPUs
> > when creating the VM.
> >
> > For now, provide a module parameter to configure the maximum number of
> > vCPUs per VM. While here, reduce the default value to 1024 to match the
> > current limit in QEMU. This number is only used by the XIVE KVM devices,
> > but some more users of KVM_MAX_VCPUS could possibly be converted.
> >
> > With this change, I could successfully run 230 mono-CPU VMs on a
> > Witherspoon system using the official skiboot-6.3.
> >
> > I could even run more VMs by using upstream skiboot containing this
> > fix, that allows to better spread interrupts between sockets:
> >
> > e97391ae2bb5 ("xive: fix return value of opal_xive_allocate_irq()")
> >
> > MAX VPCUS | MAX VMS
> > ----------+---------
> > 1024 | 255
> > 512 | 511
> > 256 | 1023 (*)
> >
> > (*) the system was barely usable because of the extreme load and
> > memory exhaustion but the VMs did start.
>
> Hrm. I don't love the idea of using a global tunable for this,
> although I guess it could have some use. It's another global system
> property that admins have to worry about.
>
Well, they have to worry only if they're unhappy with the new
1024 default FWIW.
> A better approach would seem to be a way for userspace to be able to
> hint the maximum number of cpus for a specific VM to the kernel.
>
Yes and it's mentioned in the changelog. Since this requires to add
a new API in KVM and the corresponding changes in QEMU, I was thinking
that having a way to change the limit in KVM would be an acceptable
solution for the short term.
Anyway, I'll start looking into the better approach.
> >
> > Signed-off-by: Greg Kurz <groug@xxxxxxxx>
> > ---
> > arch/powerpc/include/asm/kvm_host.h | 1 +
> > arch/powerpc/kvm/book3s_hv.c | 32 ++++++++++++++++++++++++++++++++
> > arch/powerpc/kvm/book3s_xive.c | 2 +-
> > arch/powerpc/kvm/book3s_xive_native.c | 2 +-
> > 4 files changed, 35 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> > index 6fb5fb4779e0..17582ce38788 100644
> > --- a/arch/powerpc/include/asm/kvm_host.h
> > +++ b/arch/powerpc/include/asm/kvm_host.h
> > @@ -335,6 +335,7 @@ struct kvm_arch {
> > struct kvm_nested_guest *nested_guests[KVM_MAX_NESTED_GUESTS];
> > /* This array can grow quite large, keep it at the end */
> > struct kvmppc_vcore *vcores[KVM_MAX_VCORES];
> > + unsigned int max_vcpus;
> > #endif
> > };
> >
> > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > index f8975c620f41..393d8a1ce9d8 100644
> > --- a/arch/powerpc/kvm/book3s_hv.c
> > +++ b/arch/powerpc/kvm/book3s_hv.c
> > @@ -125,6 +125,36 @@ static bool nested = true;
> > module_param(nested, bool, S_IRUGO | S_IWUSR);
> > MODULE_PARM_DESC(nested, "Enable nested virtualization (only on POWER9)");
> >
> > +#define MIN(x, y) (((x) < (y)) ? (x) : (y))
> > +
> > +static unsigned int max_vcpus = MIN(KVM_MAX_VCPUS, 1024);
> > +
> > +static int set_max_vcpus(const char *val, const struct kernel_param *kp)
> > +{
> > + unsigned int new_max_vcpus;
> > + int ret;
> > +
> > + ret = kstrtouint(val, 0, &new_max_vcpus);
> > + if (ret)
> > + return ret;
> > +
> > + if (new_max_vcpus > KVM_MAX_VCPUS)
> > + return -EINVAL;
> > +
> > + max_vcpus = new_max_vcpus;
> > +
> > + return 0;
> > +}
> > +
> > +static struct kernel_param_ops max_vcpus_ops = {
> > + .set = set_max_vcpus,
> > + .get = param_get_uint,
> > +};
> > +
> > +module_param_cb(max_vcpus, &max_vcpus_ops, &max_vcpus, S_IRUGO | S_IWUSR);
> > +MODULE_PARM_DESC(max_vcpus, "Maximum number of vCPUS per VM (max = "
> > + __stringify(KVM_MAX_VCPUS) ")");
> > +
> > static inline bool nesting_enabled(struct kvm *kvm)
> > {
> > return kvm->arch.nested_enable && kvm_is_radix(kvm);
> > @@ -4918,6 +4948,8 @@ static int kvmppc_core_init_vm_hv(struct kvm *kvm)
> > if (radix_enabled())
> > kvmhv_radix_debugfs_init(kvm);
> >
> > + kvm->arch.max_vcpus = max_vcpus;
> > +
> > return 0;
> > }
> >
> > diff --git a/arch/powerpc/kvm/book3s_xive.c b/arch/powerpc/kvm/book3s_xive.c
> > index 2ef43d037a4f..0fea31b64564 100644
> > --- a/arch/powerpc/kvm/book3s_xive.c
> > +++ b/arch/powerpc/kvm/book3s_xive.c
> > @@ -2026,7 +2026,7 @@ static int kvmppc_xive_create(struct kvm_device *dev, u32 type)
> > xive->q_page_order = xive->q_order - PAGE_SHIFT;
> >
> > /* Allocate a bunch of VPs */
> > - xive->vp_base = xive_native_alloc_vp_block(KVM_MAX_VCPUS);
> > + xive->vp_base = xive_native_alloc_vp_block(kvm->arch.max_vcpus);
> > pr_devel("VP_Base=%x\n", xive->vp_base);
> >
> > if (xive->vp_base == XIVE_INVALID_VP)
> > diff --git a/arch/powerpc/kvm/book3s_xive_native.c b/arch/powerpc/kvm/book3s_xive_native.c
> > index 84a354b90f60..20314010da56 100644
> > --- a/arch/powerpc/kvm/book3s_xive_native.c
> > +++ b/arch/powerpc/kvm/book3s_xive_native.c
> > @@ -1095,7 +1095,7 @@ static int kvmppc_xive_native_create(struct kvm_device *dev, u32 type)
> > * a default. Getting the max number of CPUs the VM was
> > * configured with would improve our usage of the XIVE VP space.
> > */
> > - xive->vp_base = xive_native_alloc_vp_block(KVM_MAX_VCPUS);
> > + xive->vp_base = xive_native_alloc_vp_block(kvm->arch.max_vcpus);
> > pr_devel("VP_Base=%x\n", xive->vp_base);
> >
> > if (xive->vp_base == XIVE_INVALID_VP)
> >
>
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