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

[Igor Mammedov <imammedo@redhat.com>]

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?

 
> Also, I tried to find a way to get the KVM VCPU max value via KVM_CAP_MAX_VCPUS ioctl,
> but the accel/kvm/kvm-all.c:kvm_init() is called later than hw/core/machine-smp.c:
> machine_parse_smp_config(), where it checks the specified QEMU -smp option.
> 
> So, if there is no objection, I will send out a patch to change m->max_cpus = 1024
> to match current KVM limit.
> 
> Best Regards,
> Suravee
>