Re: [PATCH 6/6] x86/kvm: add boot parameter for setting max number of vcpus per guest

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Juergen Gross <jgross@suse.com> writes:

> On 14.07.21 13:15, Vitaly Kuznetsov wrote:
>> Juergen Gross <jgross@suse.com> writes:
>> 
>>> Today the maximum number of vcpus of a kvm guest is set via a #define
>>> in a header file.
>>>
>>> In order to support higher vcpu numbers for guests without generally
>>> increasing the memory consumption of guests on the host especially on
>>> very large systems add a boot parameter for specifying the number of
>>> allowed vcpus for guests.
>>>
>>> The default will still be the current setting of 288. The value 0 has
>>> the special meaning to limit the number of possible vcpus to the
>>> number of possible cpus of the host.
>>>
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>> ---
>>>   Documentation/admin-guide/kernel-parameters.txt | 10 ++++++++++
>>>   arch/x86/include/asm/kvm_host.h                 |  5 ++++-
>>>   arch/x86/kvm/x86.c                              |  7 +++++++
>>>   3 files changed, 21 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>> index 99bfa53a2bbd..8eb856396ffa 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -2373,6 +2373,16 @@
>>>   			guest can't have more vcpus than the set value + 1.
>>>   			Default: 1023
>>>   
>>> +	kvm.max_vcpus=	[KVM,X86] Set the maximum allowed numbers of vcpus per
>>> +			guest. The special value 0 sets the limit to the number
>>> +			of physical cpus possible on the host (including not
>>> +			yet hotplugged cpus). Higher values will result in
>>> +			slightly higher memory consumption per guest. Depending
>>> +			on the value and the virtual topology the maximum
>>> +			allowed vcpu-id might need to be raised, too (see
>>> +			kvm.max_vcpu_id parameter).
>> 
>> I'd suggest to at least add a sanity check: 'max_vcpu_id' should always
>> be >= 'max_vcpus'. Alternatively, we can replace 'max_vcpu_id' with say
>> 'vcpu_id_to_vcpus_ratio' and set it to e.g. '4' by default.
>
> Either would be fine with me.
>
> A default of '2' for the ratio would seem more appropriate for me,
> however. A thread count per core not being a power of 2 is quite
> unlikely, and the worst case scenario for cores per socket would be
> 2^n + 1.
>

(I vaguely recall AMD EPYC had more than thread id (package id?)
encapsulated into APIC id).

Personally, I'd vote for introducing a 'ratio' parameter then so
generally users will only have to set 'kvm.max_vcpus'.

>> 
>>> +			Default: 288
>>> +
>>>   	l1tf=           [X86] Control mitigation of the L1TF vulnerability on
>>>   			      affected CPUs
>>>   
>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>> index 39cbc4b6bffb..65ae82a5d444 100644
>>> --- a/arch/x86/include/asm/kvm_host.h
>>> +++ b/arch/x86/include/asm/kvm_host.h
>>> @@ -37,7 +37,8 @@
>>>   
>>>   #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
>>>   
>>> -#define KVM_MAX_VCPUS 288
>>> +#define KVM_DEFAULT_MAX_VCPUS 288
>>> +#define KVM_MAX_VCPUS max_vcpus
>>>   #define KVM_SOFT_MAX_VCPUS 240
>>>   #define KVM_DEFAULT_MAX_VCPU_ID 1023
>>>   #define KVM_MAX_VCPU_ID max_vcpu_id
>>> @@ -1509,6 +1510,8 @@ extern u64  kvm_max_tsc_scaling_ratio;
>>>   extern u64  kvm_default_tsc_scaling_ratio;
>>>   /* bus lock detection supported? */
>>>   extern bool kvm_has_bus_lock_exit;
>>> +/* maximum number of vcpus per guest */
>>> +extern unsigned int max_vcpus;
>>>   /* maximum vcpu-id */
>>>   extern unsigned int max_vcpu_id;
>>>   /* per cpu vcpu bitmasks (disable preemption during usage) */
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index a9b0bb2221ea..888c4507504d 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -177,6 +177,10 @@ module_param(force_emulation_prefix, bool, S_IRUGO);
>>>   int __read_mostly pi_inject_timer = -1;
>>>   module_param(pi_inject_timer, bint, S_IRUGO | S_IWUSR);
>>>   
>>> +unsigned int __read_mostly max_vcpus = KVM_DEFAULT_MAX_VCPUS;
>>> +module_param(max_vcpus, uint, S_IRUGO);
>>> +EXPORT_SYMBOL_GPL(max_vcpus);
>>> +
>>>   unsigned int __read_mostly max_vcpu_id = KVM_DEFAULT_MAX_VCPU_ID;
>>>   module_param(max_vcpu_id, uint, S_IRUGO);
>>>   
>>> @@ -10648,6 +10652,9 @@ int kvm_arch_hardware_setup(void *opaque)
>>>   	if (boot_cpu_has(X86_FEATURE_XSAVES))
>>>   		rdmsrl(MSR_IA32_XSS, host_xss);
>>>   
>>> +	if (max_vcpus == 0)
>>> +		max_vcpus = num_possible_cpus();
>> 
>> Is this special case really needed? I mean 'max_vcpus' is not '0' by
>> default so whoever sets it manually probably knows how big his guests
>> are going to be anyway and it is not always obvious how many CPUs are
>> reported by 'num_possible_cpus()' (ACPI tables can be weird for example).
>
> The idea was to make it easy for anyone managing a large fleet of hosts
> and wanting to have a common setting for all of them.
>

I see. It seems to be uncommon indeed to run guests with more vCPUs than
host pCPUs so everything >= num_online_cpus() should be OK. My only
concern about num_possible_cpus() is that it is going to be hard to
explain what 'possible CPUs' mean (but whoever cares that much about
wasting memory can always set the required value manually).

> It would even be possible to use '0' as the default (probably via config
> option only).
>
>> 
>>> +
>>>   	kvm_pcpu_vcpu_mask = __alloc_percpu(KVM_VCPU_MASK_SZ,
>>>   					    sizeof(unsigned long));
>>>   	kvm_hv_vp_bitmap = __alloc_percpu(KVM_HV_VPMAP_SZ, sizeof(u64));
>> 

-- 
Vitaly