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

[Juergen Gross <jgross@suse.com>]

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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.

> 
>> +			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.

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));
> 

Thanks for the feedback,


Juergen

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