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

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Date:   Wed, 14 Jul 2021 15:04:12 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-doc@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 6/6] x86/kvm: add boot parameter for setting max number of
 vcpus per guest

On 14.07.21 13:45, Vitaly Kuznetsov wrote:
> Juergen Gross <jgross@...e.com> writes:
> 
>> On 14.07.21 13:15, Vitaly Kuznetsov wrote:
>>> Juergen Gross <jgross@...e.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@...e.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).

Ah, yes, that rings a bell.

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

Okay.

Default '4' then? Or '2 ^ (topology_levels - 2)' (assuming a
topology_level of 3 on Intel: thread/core/socket and 4 on EPYC:
thread/core/package/socket).

> 
>>>
>>>> +			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).

Right.


Juergen

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