linux-kernel - Re: [PATCH v2 6/6] x86/kvm: add boot parameter for setting max number of vcpus per guest

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Date:   Mon, 6 Sep 2021 06:47:20 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Yao Yuan <yaoyuan0329os@...il.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, maz@...nel.org, ehabkost@...hat.com,
        Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.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>
Subject: Re: [PATCH v2 6/6] x86/kvm: add boot parameter for setting max number
 of vcpus per guest

On 06.09.21 02:45, Yao Yuan wrote:
> On Fri, Sep 03, 2021 at 03:08:07PM +0200, Juergen Gross wrote:
>> 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 | 7 +++++++
>>   arch/x86/include/asm/kvm_host.h                 | 5 ++++-
>>   arch/x86/kvm/x86.c                              | 9 ++++++++-
>>   3 files changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 37e194299311..b9641c9989ef 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -2435,6 +2435,13 @@
>>            feature (tagged TLBs) on capable Intel chips.
>>            Default is 1 (enabled)
>>
>> +	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.
>> +			Default: 288
>> +
>>    kvm.vcpu_id_add_bits=
>>            [KVM,X86] The vcpu-ids of guests are sparse, as they
>>            are constructed by bit-wise concatenation of the ids of
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 6c28d0800208..a4ab387b0e1c 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -38,7 +38,8 @@
>>
>>   #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_MAX_VCPU_ID kvm_max_vcpu_id()
>>   /* memory slots that are not exposed to userspace */
>> @@ -1588,6 +1589,8 @@ 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 */
>>   unsigned int kvm_max_vcpu_id(void);
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index ff142b6dd00c..49c3d91c559e 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -188,9 +188,13 @@ module_param(pi_inject_timer, bint, S_IRUGO | S_IWUSR);
>>   static int __read_mostly vcpu_id_add_bits = -1;
>>   module_param(vcpu_id_add_bits, int, S_IRUGO);
>>
>> +unsigned int __read_mostly max_vcpus = KVM_DEFAULT_MAX_VCPUS;
>> +module_param(max_vcpus, uint, S_IRUGO);
>> +EXPORT_SYMBOL_GPL(max_vcpus);
>> +
>>   unsigned int kvm_max_vcpu_id(void)
>>   {
>> -	int n_bits = fls(KVM_MAX_VCPUS - 1);
>> +	int n_bits = fls(max_vcpus - 1);
> 
> A quesintion here: the parameter "vcpu_id_add_bits" also depends
> on the "max_vcpus", we can't calculate the "vcpu_id_add_bits" from
> "max_vcpus" because KVM has no topologically knowledge to determine
> bits needed for each socket/core/thread level, right?

Correct.


Juergen

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