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Date: Wed, 14 Jul 2021 13:45:02 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Juergen Gross <jgross@...e.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
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).
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
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