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SubjectRe: [PATCH v2 6/6] x86/kvm: add boot parameter for setting max number of vcpus per guest
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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@suse.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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