Re: [PATCH] pc: q35: Bump max_cpus to 512

[Suravee Suthikulpanit]

On 5/19/22 7:37 PM, Igor Mammedov wrote:
> On Thu, 19 May 2022 13:53:49 +0700
> Suravee Suthikulpanit<suravee.suthikulpanit@amd.com>  wrote:
>
> > On 5/13/22 6:23 PM, Michael S. Tsirkin wrote:
> > > On Mon, May 09, 2022 at 09:12:49AM +0200, Igor Mammedov wrote:
> > > > On Wed, 4 May 2022 08:16:39 -0500
> > > > Suravee Suthikulpanit<suravee.suthikulpanit@amd.com>   wrote:
> > > >   
> > > > > This is the maximum number of vCPU supported by
> > > > > the AMD x2APIC virtualization.
> > > > >
> > > > > Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@amd.com>
> > > > > ---
> > > > >    hw/i386/pc_q35.c | 2 +-
> > > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> > > > > index 302288342a..e82b1c690d 100644
> > > > > --- a/hw/i386/pc_q35.c
> > > > > +++ b/hw/i386/pc_q35.c
> > > > > @@ -357,7 +357,7 @@ static void pc_q35_machine_options(MachineClass *m)
> > > > >        machine_class_allow_dynamic_sysbus_dev(m, TYPE_INTEL_IOMMU_DEVICE);
> > > > >        machine_class_allow_dynamic_sysbus_dev(m, TYPE_RAMFB_DEVICE);
> > > > >        machine_class_allow_dynamic_sysbus_dev(m, TYPE_VMBUS_BRIDGE);
> > > > > -    m->max_cpus = 288;
> > > > > +    m->max_cpus = 512;
> > > > Maybe we should bump it to KVM VCPU maximum,
> > > > and make sure we error out if asked for combination of
> > > > hardware/irqchip is not usable.
> > > So what happens if one does 710 and then tries to use AMD x2APIC?
> > > We'd like that to error out, right?
> > >    
> > Since the AMD SVM x2AVIC can support only upto 512 vCPUs,
> > KVM would return error when QEMU tries to create more than 512 vcpus
> > in the following call-path:
> >
> >     kvm_vm_ioctl_create_vcpu
> >       kvm_arch_vcpu_create
> >         svm_vcpu_create
> >           avic_init_vcpu
> what is actual error message that end user will get from that failure?

qemu-system-x86_64: kvm_init_vcpu: kvm_get_vcpu failed (512): Invalid argument

Best Regards,
Suravee